Grease an 8-inch square baking pan with cooking spray or butter. Set aside.
In a saucepan, place the butter. Melt over low heat.
Add the marshmallows, chocolate chips, and cocoa to the saucepan and melt, stirring frequently, until the mixture is smooth.
Spray a large bowl with cooking spray. Place the Rice Krispies cereal in the bowl. Pour the marshmallow and chocolate mixture over the cereal and fold until the cereal is thoroughly coated.
Scrape the mixture into the prepared baking pan. Use a clean spatula to evenly press the treats in the pan.
Let cool for about 2 hours and cut into 2-inch squares.
Notes
To make a larger batch of these cereal treats, grease a 9 x 13-inch baking pan. Use a combination of:
16 ounces marshmallows (about 68 regular-size marshmallows OR 9 cups mini marshmallows)