Peanut Butter Crispy Bars

First melt butter and marshmallows in large saucepan over low heat. Stir frequently until melted completely. Remove from heat and stir in peanut butter until dissolved. Stir in rice cereal until the cereal is completely coated with the marshmallow mixture. Next, spread mixture into a 13 x 9 x 2-inch pan lightly coated with non-stick cooking spray. Press mixture into pan until firmly packed. Cool completely. Cut into 24 bars.