Preheat oven to 375°F. Lightly spray a mini muffin tin with non-stick cooking spray.

In a medium bowl, sift together the flour, salt and baking soda; set aside.

In a large bowl with an electric mixer on medium speed, cream together the butter, sugars, and peanut butter until light and fluffy - about 3 minutes. Add in egg, vanilla and milk. Reduce mixer speed to low and gradually add the dry ingredients into the peanut butter mixture - mixing until thoroughly combined.

Shape dough into 40 1-inch balls. Place a dough ball into each well of the prepared mini muffin pan. Take care not to press the dough balls into the wells, simply drop the dough balls into the wells.

Bake cookies in preheated oven for 8-10 minutes. Remove pan from oven and immediately press a miniature Reese's peanut butter cup into the center of each ball. Place pan on cooling rack and allow cookies to cool completely. Carefully remove cookies from pan.