Lightly spray a 9x13 pan and set aside. In a large bowl, count out the cereal then set aside. In a large pot, melt the butter over medium heat, then add the mini marshmallows. Stir constantly, until marshmallows are completely melted, to prevent scorching. Immediately pour over cereal and stir until completely covered. Add 2/3 of the Reese's Pieces to the mixture... the pieces will become all melty gooey. Spread into the 9x13 pan. If necessary, spray hands or spatula with cooking spray to prevent sticking as you press the mixture into the pan.
Pour the rest of the Pieces onto the top of the mixture, and lightly press so that they adhere to the still sticky marshmallows.