Spray a 9 × 13 inch pan with nonstick cooking spray.
In a very large and deep pan, mix together marshmallows and butter over medium heat. Stir frequently until marshmallows are melted and mixture is smooth.
Remove from heat and stir in Rice Krispies cereal and Nerds candy until completely coated.
Spread into prepared pan. Spray one hand with nonstick cooking spray and use that hand to gently press the mixture into an even layer. Do not press too hard.
Let cool completely before cutting into bars and storing in an airtight container.
Notes
Nutrition information does not include the Nerds candy.