Spray a 11×7-inch baking pan with nonstick spray. Set aside.

In a large saucepan over low heat, melt the butter. Add the marshmallows and stir constantly. Once the marshmallows are completely melted, stir in the dry cake mix. Remove pan from heat.

Stir in the cereal until it is completely coated with the marshmallow mixture. Mix in half of the sprinkles. Press into prepared baking pan and top with the remaining sprinkles. When I press rice krispie treats into a pan, I use a spatula sprayed with nonstick spray, making it easier to press down. Press down lightly, as too much pressure will create hard rice krispie treats.

Chill for at least 30 minutes and cut into squares. Squares stay fresh at room temperature for up to 1 week.