1. Remove cupcake liners (if necessary) and cut each cupcake in half horizontally; place a brownie on the bottom half of each cupcake. Set top halves aside.

2. Divide buttercream evenly between three bowls. To one bowl, add 1/4 teaspoon green food coloring and 2 drops yellow food coloring; mix well to combine. To the second bowl, add 1/2 teaspoon yellow food coloring and 1 drop red food coloring; mix well to combine. To the third bowl, add 1 teaspoon red food coloring and 1 drop blue food coloring; mix well to combine.

3. Transfer each colored buttercream to a resealable plastic bag. Snip a 1/4-inch opening from one bottom corner of each plastic bag. Using the yellow buttercream, pipe a large hollow square on each brownie. Using the red buttercream, pipe a circle in the center of the square. Top with a squiggle of the green buttercream over the red circle.

4. Brush reserved top halves of cupcakes lightly with water. Sprinkle with nonpareils, pressing to adhere. Top frosted brownies with cupcake tops to create cupcake burgers.

5. Preheat broiler.

6. Using a crinkle-cut knife, slice pound cake into 1/2-inch-thick slices. Cut each slice into a 1/2-inch-thick strip. Place pound cake strips on a baking sheet and transfer to broiler. Broil until golden and toasted, 30 seconds. Drizzle pound cake fries with red buttercream and serve with hamburger cupcakes.