one 9.5-inch diameter bowl (4 quart) glass or metal bowl; one 8-inch smaller (2.5 quart) or metal bowl (that fits inside the first bowl); one 10-inch cake board (should match the circumference of the first bowl)
Spray a 9.5-inch bowl with nonstick cooking spray. Line the bowl with plastic wrap and set aside. Turn an 8-inch bowl upside down and spray the outside with nonstick cooking spray. Cover the outside of the bowl with plastic wrap and set aside.
Prepare the vanilla mousse mix with the whole milk in another bowl according to package instructions. (The milk amount is reduced from package instructions.) Pour the mousse into the lined 9.5-inch bowl. Place the smaller bowl inside and press gently to spread the filling up the sides of the bowl until just below the rim. Put the bowls in the freezer until firmly set, at least 4 hours.
Line a baking sheet with parchment paper. Spread the watermelon cubes out onto the baking sheet in an even layer. Put in the freezer until frozen, at least 3 hours.
Combine the frozen watermelon cubes, sweetened condensed milk and red food coloring in a food processor. Pulse to break up the watermelon cubes, then let the processor run until the mixture is smooth with a texture like soft serve ice cream. Transfer the mixture to a bowl and mix in the mini chocolate chips.
Remove the bowls from the freezer. Remove the smaller bowl, revealing a well in the frozen mousse layer. Transfer the watermelon mixture into the well and spread with an offset spatula so the watermelon is smooth and is level with the mousse layer. Place a 10-inch cardboard cake board on top of the cake. Cover with plastic wrap and put in the freezer until set, at least 4 hours or overnight. This can be done up to 2 weeks in advance.
When ready to decorate, prepare the Watermelon Mirror Glaze. Fit a cooling rack into a baking sheet. Put 3 ramekins on the cooling rack so that the cake can be elevated over the rack. Remove the bowl from the freezer. Discard the plastic wrap covering the cake. Invert the bowl and remove the cake. (If you have trouble removing the cake you can place the bowl in warm water to help loosen it.) Place the cake on the ramekins on the cooling rack. Pour the green mirror glaze over the frozen cake, then squeeze on lines of the yellow mirror glaze. Put in the freezer for at least 30 minutes before serving.
Watermelon Mirror Glaze:
Combine the corn syrup and 1/3 cup water in a small pot and bring to a boil. Put the melon gelatin into a bowl. Pour the boiling corn syrup and water mixture into the gelatin and gently whisk until the gelatin is dissolved.
Combine the white chocolate and sweetened condensed milk in a double boiler and heat, stirring frequently, until the white chocolate is just melted and the mixture is smooth. Pour the gelatin mixture into the white chocolate and condensed milk mixture and slowly stir until well combined and the mixture is smooth.
Divide the glaze into 2 bowls, pouring 1/4 cup in one bowl and the remaining 3/4 cup in the second bowl. Add 1 drop of yellow gel food coloring to the bowl with 1/4 cup of the gaze and 15 drops of green gel food coloring to the bowl with the 3/4 cup of the glaze and mix until well blended. Transfer the yellow glaze to a squeeze bottle. Let sit until the temperature of the glaze is around 85 degrees F. Then it is ready to pour over the watermelon cake.
To cleanly cut the cake, add very hot water to a tall container. Place the blade of a large knife into the hot water, then wipe dry with a cloth before slicing. Continue heating the knife in between each slice.