Spray a 9x13 pan with nonstick spray. Set aside. In a large pot, melt butter and peanut butter together. Add marshmallows and stir constantly until smooth. Remove from heat and add cereal. Stir until cereal and peanut butter mixture is combined.
Pour mixture into greased pan and press down firmly into one even layer. In a microwave safe bowl, place chopped milk chocolate. Microwave in 30 second increments until smooth, stirring after each one. Pour over the peanut butter cereal in the pan. Using an offset spatula, spread into a thin, even layer.
Let the bars cool completely before cutting, about 2 hours. The cut into 16 even bars. Store in a covered storage container for up to 3 days.