Combine over hot, not boiling, water, the milk chocolate chips and vegetable shortening. Stir until the chips are melted and the mixture is smooth. Remove from the heat but keep over the hot water. Coat the inside of the candy cups by using 1 t. of the chocolate mixture for each cup. Place the candy cups in the palm of your hand and rotate gently, using a narrow rubber spatula to push the chocolate up the sides of the cup. Chill. PEANUT BUTTER FILLING: In a small bowl, combine the peanut butter, confectioners' sugar and butter, mixing until well blended. Using slightly rounded teaspoonfulls, shape the peanut butter filling into balls. Place 1 ball in each chilled cup and press lightly with your fingers to flatten. Spoon 1 level t. of the reserved chocolate mixture on top and smooth over to seal. Chill until form, about 45 minutes. Store in airtight container in the refrigerator.