Beat butter and peanut butter in large bowl until well blended.
Add 1/3 c sugar and brown sugar; beat until light and fluffy.
Add egg milk and vanilla; beat well.
Stir together flour, baking soda and salt; gradually beat into peanut butter mixture.
Cover tightly and chill for 4-8 hours in fridge.
Shape dough into 1 inch balls.
Roll in granulated sugar; place on ungreased cookie sheet.
Bake 8-10 minutes or until lightly browned.
Immediately press a chocolate piece into center of each cookie; cookie will crack slightly around edges.
Let stand on cookie for 3-4 minutes, then remove from cookie to wire rack. Cool completely. Makes 48 cookies.
To freeze, shape dough into balls but do not roll in sugar. Place on waxed paper lined cookie sheets and freeze until firm. Pack into hard sided freezer container; label, and freeze up to 3 months. To bake, roll frozen dough into sugar, then bake as directed, adding 3-6 minutes to the baking time.