Preheat oven to 375 degrees F
Using an electric hand mixer, cream together the butter, white sugar, brown sugar, and peanut butter until smooth. Beat in the egg, vanilla, and milk
In a separate bowl mix together the flour, salt, and baking soda
Add the flour mixture to the peanut butter mixture and mix well until all combined
Shape into 40 balls and place each into an ungreased mini muffin pan
Bake in preheated oven for 8-9 minutes. Remove pan from oven and immediately press a mini peanut butter cup into each ball. Cool and carefully remove from pan