Watermelon Lime Cake

May 19

Tender white cake layers with watermelon simple syrup, watermelon buttercream and lime curd. 

Who else is so ready for summer? Our winter in Utah has trickle into May, which is totally not ok with me! To take my mind off the icky weather, I’ve been thinking about all the summer-inspired cakes I want to make. The list continues to grow, so before it gets out of control, I decided to get started with this Watermelon Lime Cake.

I wanted to do this cake for a few reasons: 1. watermelon is the fruit of the summer (in my opinion, that is – I could eat all day, everyday), 2. I’ve seen dozens of watermelon cakes around Pinterest but none of them seem to have a watermelon flavor, just the design, and 3. I wanted to do my own version of a watermelon design. As I thought about the cake, I had a hard time settling for just the watermelon flavor by itself. A flavor pairing was calling out to me. The missing part? Lime curd! I immediately felt like the tartness of the lime would contrast and compliment the watermelon flavor so well.

I immediately got to work, but this cake didn’t come together as quickly as some of my other cakes. The cake layers took me a few tries to nail. In my first attempt, I used watermelon Jell-O and my white cake recipe for a base. The batter tasted great but the watermelon flavor completely disappeared during baking. In my second attempt, I used the Jell-O mix again and watermelon puree instead of buttermilk as the liquid. Again, the flavor of the batter was great but I lost it during baking. I thought about using the LorAnns brand of watermelon candy flavoring but after a taste test with my husband and son, we decided it tasted too much like bubble gum. Scratch that idea!

I needed to stick with watermelon Jell-O… but I had to come up with another way for the flavor to stand out more.

I decided to think about it some more and after a good night’s rest, I literally woke up at 6 a.m. with the solution in my head. (I must have worked it all out in my dreams! Wish that was the case with more than just my cake problems). 

Instead of incorporating the flavor into the batter, I would incorporate the flavor into the cake… after it was baked. The solution? A watermelon simple syrup, made with water, sugar and watermelon Jell-O.

The watermelon simple syrup turned out perfectly, and added to the baked cake layers, gave me the flavor I was looking for.

I also added watermelon flavor to the buttercream – again, using the Jell-O (the candy flavoring wasn’t right and watermelon puree wouldn’t hold up in a buttercream). I love the flavor of the buttercream, especially paired with the cake layers and lime curd. The one thing you’ll probably notice is that the Jell-O doesn’t completely dissolve. I actually didn’t have a problem with this because I barely noticed it when I was eating the cake, only when I sampled the frosting on its own. I also don’t mind the small granules because they create the cutest speckled look on the cake once you add the food coloring.

Overall, I’m so happy with how this cake turned out – both the flavor and the design are exactly what I was going for.

My mom was actually in town when I was going through all my testing and had the chance to sample everything along the way, as well as the final product.  She left about a week ago and keeps texting me to tell me how much she loved this cake and how perfectly the flavors go together. (It literally makes me giddy when people can’t forget the way my cakes taste! Best compliment ever!). 

This truly is a memorable cake and so perfect for summer. I hope you love it!

Watermelon Lime Cake

Yield 1 4-layer, 6-inch cake

Tender white cake layers with watermelon simple syrup, watermelon buttercream and lime curd.


For the Cake

  • 3 cups cake flour
  • 1 3/4 cup granulated sugar
  • 4 1/2 teaspoons baking powder
  • 1 teaspoon salt
  • 1 cup unsalted butter, cut into cubes, room temperature
  • 1 1/4 cup buttermilk, divided, room temperature
  • 5 egg whites, room temperature
  • 1 teaspoon clear vanilla extract
  • Pink food gel, optional

For the Watermelon Simple Syrup

  • 1/2 cup water
  • 1/2 cup sugar
  • 1 (3 oz.) package of watermelon Jell-O

For the Watermelon Buttercream

  • 2 cups unsalted butter, room temperature
  • 6 cups powdered sugar, sifted
  • 3 tablespoons heavy cream
  • 2 teaspoons clear vanilla extract
  • 1 (3 oz.) package of watermelon Jell-O
  • Pinch of salt

For the Lime Curd

  • 3/4 cup granulated sugar
  • 4 teaspoons lime zest
  • 1/2 cup fresh lime juice
  • 3 egg yolks and 3 whole eggs
  • 4 tablespoons cold unsalted butter, cut into cubes


  • Mini chocolate chips


For the Cake

  1. Preheat your oven to 350 degrees F. Prepare four 6-inch cake pans by spraying each pan with nonstick spray, lining the bottoms with parchment rounds and spraying the pans again. Set aside.
  2. In the bowl of a stand mixer fitted with the paddle attachment, combine the flour, sugar, baking powder and salt. Stir to combine.
  3. With the mixer on low gradually add the butter and 1 cup of the buttermilk. Turn the mixer to medium-low and stir until the ingredients are combined, about 30 seconds. 
  4. In a small bowl, combine the egg whites, 1/4 cup of buttermilk and the clear vanilla extract. Whisk to combine.
  5. With the mixer on low speed, add the egg white mixture to the batter in three batches, scraping down the sides of the bowl after each addition. 
  6. Evenly divide the batter into the prepared pans (about 10 to 11 ounces of batter in each of the four 6-inch pans).
  7. Bake the cake layers for 20 to 23 minutes, until a toothpick inserted in the center comes out clean or with only a few crumbs on it. Let cool in the pans for about 10 minutes before inverting onto cooling racks to cool completely .
  8. After the cake layers have cooled completely, level each layer with a cake leveler or serrated knife. At this point, you can apply the watermelon simple syrup and begin to stack the cake or after you apply the watermelon simple syrup, wrap each cake layer individually with plastic wrap and freeze for up to a week. If you want to freeze the cake layers longer, add an additional layer of plastic wrap and then tin foil around each cake layer. Allow the cake layers to thaw for about an hour before assembly,

For the Watermelon Simple Syrup

  1. In a small sauce pan, combine the sugar and water. Bring the mixture to a boil, stirring occasionally, until the sugar has completely dissolved in the water. Remove from the heat and stir in the watermelon Jell-O, stirring constantly until the Jell-O has dissolved.
  2. Allow the mixture to cool slightly before using a pastry brush or squeeze bottle to apply it onto the cake layers.

For the Watermelon Buttercream

  1. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium-high speed for about 2 minutes, until smooth and slightly lighter in color.
  2. Turn the mixer to low and gradually add the powdered sugar, followed by the Jell-O packet.
  3. Once all the powdered and Jell-O are incorporated, add the heavy cream, vanilla and pinch of salt. Turn the mixer to medium-high and beat for about 5 minutes. There will still be small Jell-O sugar granules in the frosting, but they’re hard to notice when you taste the buttercream and it actually adds a little speckled look to the frosting once you add food coloring.
  4. Save 1 1/2 cups frosting for the center layers and crumb coat of the cake. You don’t need to add food coloring to this portion.
  5. Reserve 1/3 cup of the frosting for the dark green, 1/4 cup of the frosting for the light green, 1/4 cup for the white, and the rest for the darker pink.

For the Lime Curd

  1. In a small sauce pan, combine the sugar, lime juice, and lime zest over medium heat. Stir until the sugar is dissolved. Add the egg yolks and whole eggs, stirring while you add them. Continue to stir while the mixture cooks for about 5 to 6 minutes. The mixture will thicken during this time. Remove it from the heat and add the butter. Stir until smooth.
  2. Run the curd through a fine mesh sieve. Once cooled, place plastic wrap directly on top of the curd and refrigerate for at least 4 hours.


  1. Place the first cake layer, top side up, on a cake board. Cover the cake layer with 1/4 cup frosting and pipe a rim around the edge of the cake. Fill the center of the cake layer with 2 to 3 tablespoons lime curd. 
  2. Continue to stack and fill the cake, following step 1, with the second and third cake layer. Place the final cake layer, top side down on top of the last layer of curd. 
  3. Using the remaining light pink buttercream you reserved for the filling and crumb coat, apply a thin layer of frosting around the cake to lock in the crumbs and lime curd. Place the cake in the freezer for 10 minutes while you tint the frosting for the watermelon design.
  4. I used about 2 drops of mint green Americolor food gel for the dark green and 1 drop for the light green. I then used about 3 drops of watermelon Americolor food gel for the dark pink and kept a little bit of the original frosting color un-tinted for one layer of the piping as well.
  5. After the crumb coat is set, pipe three lines of dark green frosting around the bottom of the cake, followed by one line of light green frosting, one line of white frosting and then the rest in dark pink.
  6. Using a cake scraper, scrape the frosting around the cake to smooth out the lines. Add chocolate chips to the dark pink frosting for "seeds".

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