With a mixer, cream together the peanut butter, butter, sugars until blended.
Beat in the egg, milk, and vanilla.
Combine the flour, baking powder and salt in a separate bowl.
Add flour mixture to the peanut butter mixture and mix together. Scoop tablespoons of dough and roll into balls.
Place cookies 2 inches apart on parchment lined cookie sheets. Bake for 8-10 minutes at 375 degrees. Don't over cook! These cookies are much better soft and just barely cooked all the way through. Bake for 9 minutes.
Immediately after removing the cookies from the oven, gently press a Reese's thin Peanut Butter Cup into the center of each cookie. Allow the cookies to cool and the chocolate to set up before serving.