Spray a 13x9 baking dish with cooking oil spray. In a large bowl, combine peanut butter, butter, powdered sugar, and graham cracker crumbs. Dump mixture into greased baking dish, and pat out mixture evenly all the way to the corners.
In a microwave-safe bowl, melt milk chocolate chips by microwaving on high for 30 seconds at a time, stopping to stir in between. Once melted, pour all over flattened peanut butter mixture, spreading chocolate evenly into one layer with a butter knife or spatula. Allow to cool and setup for 2 hours (or cool faster in the refrigerator). Cut into parts and put in cupcake papers for saving. Makes lots. Enjoy!