Heat the oven to 375°F.
Beat the butter, peanut butter, ½ cup granulated sugar and brown sugar until creamy. Beat in the egg, milk and vanilla.
Whisk the flour, baking soda and salt in a separate bowl, then stir into the peanut butter mixture until a soft dough forms. If it's too sticky to roll, chill it for 30 minutes.
Shape the dough into 1-inch balls and roll in sugar. Place 2 inches apart on ungreased cookie sheets.
Bake 10 to 12 minutes, until light golden brown. Press a kiss into the center of each cookie right away.
Cool completely, until the chocolate is set.