PREHEAT oven to 375°F. Beat butter, peanut butter, 1/3 cup of the granulated sugar and the brown sugar in large bowl with electric mixer on medium speed until light and fluffy. Add egg, milk and vanilla; beat until well blended. Combine flour, peanuts, baking soda and salt. Gradually add to butter mixture, beating until well blended after each addition.
SHAPE dough into 48 balls, each about 1 inch in diameter; roll in remaining 1/3 cup granulated sugar. Place, 2 inches apart, on ungreased baking sheets.
BAKE 9 to 10 min. or until lightly browned. Immediately press a chocolate piece into center of each cookie. (Cookie will crack slightly around edge.) Cool on baking sheets 1 min. Remove to wire racks; cool completely.