Preheat oven to 375 degrees F
Using an electric hand mixer cream together softened butter, peanut butter, brown sugar, and 1/2 cup white sugar until smooth. Add egg, milk, and vanilla extract and beat well
Stir together flour, baking soda, and salt. Add to creamed mixture and beat on low speed until stiff dough forms
Shape into 1 inch balls. Roll each ball into white sugar. Place each ball 2 inches apart on an ungreased cookie sheet
Bake in preheated oven for 10-12 minutes or until golden brown
Remove from oven and top each cookie immediately with an unwrapped Hershey Kiss chocolate, pressing down just until the cookie cracks around the edge. Place cookies on wire rack to cool completely