melt butter in large saucepan over med-low heat, stir in marshmallows. stir until melted. add peanut butter until combined, remove from heat. stir in rice krispies. i just pour directly from the box, so i don't have exact measurements. i would start with 6 cups and add more as needed. it shouldnt be too gooey. press into greased 13x9 pan. cool completely and cut.