Coat a standard 12-cup muffin pan with cooking spray. Set aside.

Melt butter in a large saucepan over medium heat. Add marshmallows, stirring to combine completely with butter.
 
Remove the saucepan from the heat and immediately add the crispy rice cereal. Working quickly, mix until well combined.

Place about 1/2 cup cereal mixture into each muffin well. Use a ¼- cup measuring cup to make an indentation in the middle of each muffin cup to form cups that are about ½-inch thick on the bottom and the sides.

Remove the cereal cups from the muffin tin, and refrigerate or freeze if you’re not planning to eat them within 24 hours. 
 
To serve, fill cups with ½ cup ice cream and garnish with a chocolate cookie.