Spray a 9 x 13″ pan with cooking spray and set aside.
In a large microwave-safe boil, heat marshmallows and butter for 90-120 seconds, stirring every 30 seconds, until melted and smooth. Stir in vanilla.
If the bowl is big enough, stir in cereal until evenly coated. If it’s not big enough, pour cereal into an even bigger bowl, pour marshmallow mixture over the top, and stir until coated.
Stir in Reese’s Pieces, and pour cereal mixture into prepared baking sheet. Spray one side of a sheet of wax paper with cooking spray, and use the sprayed side to press the cereal evenly into the pan. Allow to set up for at least two hours (4-6 hours is better) before cutting into squares and serving.