Line three baking sheets with parchment paper. Set aside.

In the bowl of your electric mixer (or with a hand mixer), cream the butter. Add the peanut butter and sugars and beat until light and fluffy (about 2 - 3 minutes). Add the egg and vanilla extract and beat to combine. Beat in the milk. In a separate bowl whisk together the flour, baking soda, and salt. Add to the peanut butter mixture and beat until incorporated. Cover and chill the batter for about an hour, or until firm enough to roll into balls.

Preheat oven to 375 degrees F (190 degrees C) and place rack in the center of the oven. 

Roll the batter into 1 inch (2.54) round balls. Place the granulated white sugar in a shallow bowl and roll each ball in the sugar. Place on the prepared baking sheet, spacing about 2 inches (5 cm) apart.

Bake the cookies for about 8 - 10 minutes, or until the cookies are lightly browned. Immediately upon removing the cookies from the oven, place a chocolate Kiss in the center of each cookie, pressing down until the cookie just starts to crack. Cool completely on a wire rack. 

Makes about 4 dozen cookies.