Preheat your oven to 375°F. Sift the all-purpose flour, salt, and baking soda together in a bowl; set aside. Place peanut butter cups in the freezer temporarily until after you bake the cookies.

In a separate bowl, cream the softened butter, granulated sugar, brown sugar, and peanut butter until it becomes light and fluffy.

Beat in the egg, vanilla extract, and milk until the mixture is smooth.

Incorporate the sifted dry ingredients until they are just combined. If the dough appears dry, add an additional tablespoon of milk. Be careful not to overmix.

Form the dough into 1-inch balls and arrange them in an ungreased mini muffin pan. Bake for 7 to 9 minutes or until thoroughly baked.

When removed from the oven, immediately press a frozen miniature peanut butter cup into the center of each cookie. Let the cookies cool completely before removing them from the pan.