In a large saucepan, combine peanut butter, granulated sugar, brown sugar, and milk. Cook over medium heat, stirring constantly, until melted and smooth.
Remove from heat and stir in powdered sugar and Rice Krispies cereal until well combined.
Press mixture into a greased 8x8 inch baking pan.
Refrigerate for at least 2 hours before cutting into bars.
Notes
Store leftover bars in an airtight container in the refrigerator.