Gather all ingredients and tools.
In a large saucepan, melt the butter over low heat.
Add the peanut butter and stir until smooth.
Stir in the marshmallows and continue to heat until melted.
Remove from heat and add Rice Krispies cereal.
Mix thoroughly until all cereal is coated.
Line a 9x13-inch baking dish with parchment paper.
Pour the mixture into the dish and press down firmly.
Allow to cool for about 30 minutes.
Cut into squares and serve.