In a large bowl, cream the butter, peanut butter and sugars until light and fluffy. Beat in the egg yolks, milk and vanilla. Combine the flour, cocoa and baking soda; gradually add to creamed mixture and mix well. Stir in chips and peanut butter cups.
Drop heaping tablespoonfuls 2 in. apart onto ungreased baking sheets. Bake at 350° for 8-10 minutes or until set (do not overbake). Cool for 2 minutes before removing from pans to wire racks. Store in an airtight container. Yield: 4 dozen.