Preheat oven to 375 degrees.
Spray mini muffin tin with cooking spray & set aside.
In a small bowl, combine flour, baking soda, & salt.
In large mixer bowl, cream together butter, granulated sugar, brown sugar & peanut butter until fluffy.
Beat in egg, vanilla & milk.
Slowly add dry ingredients. Mix well.
Roll dough into balls & place in prepared pan. Leave in ball form- do not flatten
Bake 7-8 minutes. They should look puffy, not browned on the sides.
Immediately after removing from the oven, place one peanut butter cups in the center of each cookie cup & press down into the cup.
Allow to fully cool before removing from pan as the peanut butter cups will be gooey and the cookie cups need to set to hold their shape.