If you want to use black fondant to make seeds for the top of the cake, pinch off a tiny piece of fondant and roll it between your fingers to make it round. Press down to flatten it, then pinch the tip so it forms a seed shape. Repeat until you’ve made about 20 seeds. (If you don’t want to do this, you can substitute chocolate chips or mini chips instead.)
Place one cake round on an 8” cake cardboard, and put it on a cake turntable. Add about a cup of red buttercream on top, and spread it out to the edges of the cake in an even layer. Scatter some miniature chocolate chips on top of the frosting, and use a spatula to swirl them into the buttercream.
Place a second cake layer on top of the first, and repeat the frosting/chocolate chip/cake pattern until you’ve stacked all of the cake layers.
Spread a layer of white buttercream all around the edges of the cake, leaving the top plain. The white will look like the watermelon’s rind when the cake is cut. Add enough buttercream so that the cake edges are completely covered and not peeking through. Refrigerate the cake for about 30 minutes to firm up the frosting. Place about 2/3 cup of white buttercream into a pastry bag fitted with a ½-inch round tip, and set aside.
Tint the remaining white buttercream with a few drops of yellow and green food coloring, to make a light yellowish-green color.
Spread this green buttercream in a thin layer over the white buttercream, just enough to cover it smoothly. Take the white buttercream in the piping bag, and pipe a ring of white frosting around the top of the cake, staying close to the edge.
Put the remaining red buttercream on top of the cake, and spread the red out until it reaches the white frosting circle. Press your black fondant seeds (or chocolate chips) into the red frosting. Chill the cake well, for at least an hour, until the sides are very firm.
Now for the fun part: painting the sides! I recommend wearing gloves for this step, unless you enjoy having bright green fingers. Mix together a teaspoon of green coloring with a tablespoon of vodka. Take a soft, new natural sponge and get it damp, then dip it into the food coloring. Dab the sponge on the side of the chilled cake in a vertical line, making a thick stripe. Make a second stripe, leaving a little space in between the two. Repeat around the sides of the cake, mixing together more food coloring and vodka if necessary, until you have green stripes all the way around.
Rinse the sponge, and mix together yellow food coloring and vodka in the same way. Repeat the same procedure, adding yellow stripes in between the green ones. Dab the green and yellow together, to make them blend and make the stripes look more organic and less precise. Work the colors on top of each other, and don’t be afraid to add darker patches of green and yellow—the more random it is, the more natural it will look!
For the best taste and texture, serve this cake at room temperature.