In a large mixer bowl, beat butter or margarine and peanut butter with electric mixer on medium speed about 30 seconds. Add the 1/2 cup sugar and the brown sugar and beat till fluffy. Add egg, milk and vanilla. Beat well.
In a medium mixing bowl stir together flour, baking powder, salt and baking soda. With mixer on low speed, gradually add flour mixture to peanut butter mixture, beating well. If necessary, cover and chill about 1 hour for easier handling.
Turn oven to 375. Shape dough into 1-inch balls. Roll in additional sugar. Place about 2 inches apart on ungreased cookie sheets. Bake 10 to 12 minutes or till edges are firm. Immediately press a chocolate kiss atop each cookie. With a pancake turner lift cookies onto cooling rack.