Instructions

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. Add pink food gel, if using, and mix on low until combined.
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 (directions below) 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 in a small saucepan, heat the water and Jell-O until Jell-O dissolves and thickens slightly. Let cool to room temperature.
2. 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.
3. Turn the mixer to low and gradually add the powdered sugar. Slowly stream in the Jell-O.
4. 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. If the frosting gets too soft from the warm Jell-O syrup, refrigerate for 15 minutes or so. 
5. 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.
6. Reserve 1/3 cup of the frosting for the dark 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.

Assembly

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. If the cake feels wobbly, place a couple plastic straws in the cake to act as dowels and keep the cake from sliding around as you apply the crumb coat.
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".