In a mixing bowl, cream butter, peanut butter and sugars. Beat in egg, milk and vanilla. Combine the flour, baking soda and salt; add to creamed mixture and mix well. Stir in chocolate chips.
Roll into 1 inch balls; roll in additional sugar. Place 2 inches apart on parchment lined baking sheet. Flatten if desired. Bake at 350 for 8-10 minutes or until golden brown. Remove to wire racks to cool. Makes 4 dozen.