These Peanut Butter Cookies are peanut butter cookie cups stuffed with a mini Reese's peanut butter cup.
Cream together the butter, sugar, peanut butter and brown sugar until fluffy. Beat in the egg, vanilla and milk. Add the flour mixture; mix well. Shape into 1 inch balls and place each into a greased mini muffin tin (or use paper liners).
Bake at 375 degrees for about 8 minutes. Do not overbake! Remove from oven and immediately press a mini peanut butter cup into each ball. Cool and carefully remove from pan.
Source: Allrecipes: http://allrecipes.com/recipe/9471/peanut-butter-cup-cookies/