Preheat the oven to 350 degrees F. Spray a 2 1/2-quart ovenproof bowl and two 8-inch-round cake pans with cooking spray. Beat the cake mix, eggs, buttermilk and vegetable oil in a large bowl with a mixer.
Divide the batter among the bowl and pans; bake until a toothpick comes out clean, about 25 minutes for the pans and about 45 minutes for the bowl. Cool in the pans 15 minutes, then unmold onto racks.
Make the lettuce: Sprinkle some green or white sugar on a cutting board. Roll out the spearmint candies to 1/8 inch thick with a rolling pin, sprinkling with more sugar to keep the candies from sticking.
Cut each flattened gummy piece into 3 strips. Fold each strip accordion-style to make ruffles; pinch the ends together.
Make the cheese: Roll out the orange fondant on a cornstarch-dusted surface to about 1/8 inch thick. Trim the fondant into a 7-inch square; cover with plastic wrap and set aside.
Trim the top of 1 round cake and the flat side of the bowl cake to make them level; crumble the trimmings. Mix the vanilla frosting, 4 tablespoons chocolate frosting and the yellow and red food coloring in a bowl to make a light brown frosting.
Make the bottom bun: Put the trimmed round cake on a serving plate or cake board. Frost with some of the light brown frosting. Arrange the gummy-candy ruffles around the edge of the cake.
Make the patty: Spread the remaining chocolate frosting over the top and sides of the untrimmed round cake. Press the crumbled cake trimmings into the frosting.
Assemble the burger: Put the chocolate patty on the bottom bun, then drape the orange fondant on top. Frost the bowl-shaped cake with the remaining light brown frosting; place on the patty. Press the rice cereal into the frosting.