Stir together peanut butter and butter in a bowl with a wooden spoon. Add sugar; work into a smooth dough with your hands. Refrigerate 30 minutes.
Using the palms of your hands, roll dough into 1 1/2-inch oblong "acorns" (you should have 36), flatten 1 end of each a bit with your fingertip, and place on a waxed paper-lined rimmed baking sheet. Refrigerate 30 minutes.
Meanwhile, melt chocolate in a bowl set over a saucepan of simmering water. Working with 1 acorn at a time, insert a toothpick into rounded tip, dip flat end into chocolate, then return to baking sheet. Remove toothpick, and insert a pretzel piece into the chocolate-dipped end of each acorn. Refrigerate until chocolate is set, about 2 hours.