Use a mixer to cream together butter, peanut butter, and sugars. Add egg, milk, and vanilla.
In a separate bowl, combine flour, baking soda, and salt. Slowly beat in to wet mixture.
Place dough in refrigerator and let chill for an hour.
Remove dough and shape into 1 1/2 inch balls. Roll in sugar and place on greased cookie sheet 2 inches apart.
Bake at 375 for 10 minutes. Remove from oven and press one Reese's heart into the center of each cookie.