Heat oven to 375 degrees. Prepare a miniature muffin tin pan and spray with cooking spray. Set aside.
In a small mixing bowl, add the flour, baking soda, and salt. Stir with a whisk to break up any clumps. Set aside.
In a larger mixing bowl, cream together the butter, granulated sugar, brown sugar, and peanut butter. Beat for 1-2 minutes until lighter in color and fluffy looking.
Add vanilla, egg, and milk. Beat until combined. Stir in the dry ingredients and mix by hand, with a wooden spoon just until combined.
Roll the cookie dough into 1" dough balls and place one in each miniature muffin tin. Cook for 7-8 minutes. Cookie cups will look puffy and lighter in color, but should NOT be browned at all on the sides.
Immediately after taking cookie cups out of the oven, gently press an unwrapped miniature Reese's peanut butter cup into the center of each one. Let cool in the pan for 10-15 minutes and then transfer to a wire rack to cool completely.