1. Heat oven to 375°F. Remove wrappers from candies. Line small muffin cups (1-3/4 inches in diameter) with paper bake cups.
2. Beat butter, brown sugar, granulated sugar, peanut butter, egg and vanilla until light and fluffy in large bowl. Stir together flour, baking soda and salt; add to butter mixture, beating until well blended. Shape dough into 1-inch balls; place one in each prepared muffin cup. Do not flatten.
3. Bake 10 to 12 minutes until puffed and lightly browned; remove from oven. Immediately press peanut butter cup or piece onto each cookie. Cool completely in muffin pan. About 3-1/2 dozen cookies.