Melt butter. Add peanut butter. Stir. Remove from heat. Add powdered sugar a little at a time, stirring in until smooth. Press into 8 x 20 cm pan. Melt chocolate chips in microwave oven on high, stirring after 1 minute and checkin g every 30 seconds thereafter. Stir well and spread on top of peanut butter mi xture. Let stand to cool or chill. Cut into squares. Makes 20.