Preheat oven to 350˚F. Grease two 9-inch cake pans and line bottom with parchment paper. Set aside.
Puree the watermelon cubes and measure out one cup of watermelon puree.
Put the cake mix, watermelon puree, eggs, vegetable oil, and watermelon Kool-Aid (if using) in a large mixing bowl. Mix for 30 seconds on low and then beat for 2 minutes at medium speed.
Divide batter between two cake pans.
Bake at 350˚F for 29-33 minutes. Cool for 20 minutes in the pan, remove the cakes from the pan and then cool completely on a rack.
Tint about 1 1/2 cups of the buttercream frosting pink, about 3/4 cup green, and leave 1/4 cup white.
Place one cake on a cake round or cake stand. Frost top with about 3/4 cup of pink frosting. Add the second cake and frost the outside edges with green buttercream and then the top with the remaining pink buttercream. Put the white buttercream in a pastry bag (or ziptop bag) and snip the corner. Pipe the white frosting along the edge of the cake.
Press chocolate chips into the top of the cake.
Video
Notes
One: 9x13-inch Cake
Pour the batter into a greased 9×13-inch pan and bake for 34-38 minutes.
Two: Storage
Place the cake in an airtight container, or cover it in plastic wrap. Then, store it in the fridge for 3-4 days.