For the Cake
Preheat your oven to 325 degrees F. Prepare four 6-inch or three 8-inch cake pans by spraying each pan with nonstick spray, lining the bottoms with parchment rounds and spraying the pans again. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and the sugar until light and fluffy, about 2 minutes.
Add the egg whites, one at a time, mixing well between each addition, followed by the vanilla. Once combined, mix on medium-high for 1 minute. Add a drop of pink food gel and stir to combine.
In a medium bowl, combine the flour, the baking powder, and the salt. Whisk to combine.
With the mixer on low speed, alternately add the flour mixture with the buttermilk mixture to the butter mixture in your large mixing bowl.
Evenly divide the batter into the prepared pans (about 10 to 11 ounces of batter in each of the four 6-inch pans or 15 ounces in each 8-inch pan).
Bake the cake layers for 30 to 33 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 .
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,