Begin by greasing a 9x13 inch baking pan with a little butter or cooking spray to prevent the bars from sticking. Set the pan aside.

In a large saucepan, place 4 tablespoons of butter and set it over medium heat. Stir occasionally until the butter is completely melted and bubbly, about 2-3 minutes.

Once the butter is melted, add 10 ounces of mini marshmallows to the saucepan. Keep stirring constantly for about 3-5 minutes until the marshmallows are gooey and fluffy.

Stir in 1/2 cup of peanut butter into the melted marshmallow mixture and mix until well-combined and smooth, about 2 minutes.

Remove the saucepan from the heat, then quickly fold in the 13-ounce box of Reese’s Puffs cereal until thoroughly coated.

Carefully pour the cereal mixture into the prepared pan. Press the mixture evenly into the pan using wet hands or a buttered spoon.

Allow the bars to cool at room temperature for about 30 minutes. Once firm, cut the bars into squares or rectangles.