Heat oven to 375°. Remove wrappers from candies. Line small muffin cups (1 ¾ inches in diameter) with paper bake cups, if desired. Beat butter, granulated sugar, brown sugar, peanut butter, egg and vanilla until fluffy in a large bowl. Stir together flour, baking soda and salt; gradually add to butter mixture, beating until well blended. Shape dough into 1 inch balls; place one ball in each paper-lined or ungreased muffin cup. Do not flatten. Bake 10 to 12 minutes until puffed and lightly browned; remove from oven. Immediately press peanut butter cup in center of each cookie. Cool completely in muffin pan.