In a large mixing bowl, cream butter and sugars for 4 minutes or until light and fluffy. This ensures the texture is perfectly creamy and no graininess from the sugar is left.
Add peanut butter, egg, milk, and vanilla and cream for 2 minutes longer scraping the sides of the bowl often.
Stir in flour, baking soda, and salt and mix until just combined. If time is on your side, let chill for 30 minutes.
Preheat oven to 375 degrees. Roll into 1-inch balls and roll into sugar. Bake for 8-10 minutes or just until the cookies are cooked through but still soft and chewy.
While the cookies are baking, unwrap the Hershey Kisses so you are ready to put them on the hot cookies. While they are still warm, press a Hershey kiss into each peanut butter cookie. Allow to cool before removing from cookie sheet.