In a saucepan, melt together peanut butter and butter or margarine. Add the icing sugar and graham cracker crumbs. Blend together. Pour and press into a 9"x13" pan. Melt the butter and chocolate chips together. Spread the mixture on top of the squares.
Put the squares in the refrigerator to firm up.
To serve, take them out of the fridge and leave them at room temperature for a while before cutting so the chocolate doesn't crack. Makes up to 36 squares.