Special equipment: two 8-inch cake pans and a 2-quart (8-inch) metal bowl; parchment
Preheat the oven to 350 degrees F. Spray two 8-inch cake pans and a 2-quart (8-inch) metal bowl with cooking spray and coat with flour, tapping out the excess. Line the bottoms of the cake pans with circles of parchment.
Toss the chocolate chips with 1 tablespoon of the flour in a small bowl and set aside.
In a stand mixer fitted with the paddle attachment, combine the cake mix, eggs, vegetable oil, the remaining flour and 2 cups cold water. Beat on low speed just to combine, then increase the speed to high and beat until very smooth, about 2 minutes. Add red food coloring to deepen the shade from light to dark pink (15 to 20 drops). Stir the flour-coated chocolate chips into the batter. Pour half of the batter into the bowl and divide the remaining batter between the 2 cake pans.
Bake all 3 cakes until the cake rounds (not the bowl) are risen and a tester inserted in the center comes out clean, 23 to 25 minutes. Remove the cake rounds to a cooling rack, reduce the oven temperature to 300 degrees F and continue to bake the cake in the bowl until it is risen and a tester inserted in the center comes out clean, 40 to 45 minutes more. Cool the bowl cake on a rack, then unmold all three cakes and remove the parchment from the rounds.
With a serrated knife, cut the domed tops from all 3 cakes to flatten them, and dust off any excess crumbs.
Remove a heaping 1/4 cup of the frosting to a small bowl and stir in enough red food coloring to approximately match the color of the inside of the cake. Place 1 cake round, cut-side up, on a serving plate. Spread a very thin layer of red frosting on the layer. You do not want the layer to be visible when you cut the cake, it is just to glue the layers together. Top with the second cake round, cut-side down. Spread with another very thin layer of red frosting. Top with the domed cake.
Frost the entire cake with a layer of white frosting about 1/4 inch thick to make the watermelon "rind." Let the frosting dry and harden in the refrigerator for about 20 minutes
Meanwhile, put the remaining frosting in an electric mixer fitted with the paddle attachment and add green food coloring, about 15 drops to start, to make a medium-pale green color several shades lighter than your fruit candy.
Smooth the green frosting over the white layer, taking care not to combine the two colors. Drape the green fruit candy over the cake, cutting to fit as necessary, starting in the center like spokes in a wheel to make the lines on the watermelon. Top with the black or brown licorice candy as a stem.
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