Coat a 9x13 inch baking pan with cooking spray. Set aside.
In a large saucepan, melt butter over medium heat until just browned. Stir the butter occasionally as it foams, crackles, and turns brown. Add the marshmallows and peanut butter and stir with a wooden spoon or spatula until smooth and completely melted. Remove the pan from the heat and add the cereal. Stir quickly, coating the cereal entirely in the marshmallow mixture.
Pour the mixture into the prepared pan and smooth out with a spatula. Add a coating of sprinkles to the top and allow to cool completely before cutting. The bars will keep, well wrapped, at room temperature for up to 4 days.