In a medium bowl, combine flour, baking soda and salt. Set aside.
Cream butter, sugars and peanut butter together until creamy. Continue beating and add in egg, extract and milk. Turn your mixer speed down and gradually add the flour mixture. Mix until well combined.
Roll into 40 balls using the palms of your hands. Place each ball into its own ungreased mini-muffin pan cup.
Bake for 8 minutes. Immediately press a Reese's cup into the center of each ball. Let cool on wire rack then remove from pan.