Ingredients:
- Baking spray with flour
- 2 (15 1/4-oz.) pkg. devil's food cake mix (such as Duncan Hines)
- 4 cups (2 lb.) unsalted butter, at room temperature
- 1 (2-lb.) pkg. powdered sugar
- 1 teaspoon vanilla extract
- ½ teaspoon kosher salt
- 1 (0.67-oz.) tube green food coloring gel
- ½ teaspoon blue food coloring gel, or more to preference
- ¼ teaspoon yellow food coloring gel, or more to preference
- ½ cup candy-coated chocolate pieces (such as M&Ms) (from 1 [3.14-oz.] pkg.)
- 1 cup popped popcorn