1. Heat oven to 350 F. Grease 8- or 9-inch square baking pan.
2. Beat peanut butter, butter, brown sugar and granulated sugar in large bowl until well blended. Add egg, milk and vanilla; beat well. Stir together flour, baking soda and salt; add to peanut butter mixture, beating until well blended.
3. Spread one half of batter in prepared pan. Sprinkle mini peanut butter cups over batter. Drop remaining batter by 1/2 teaspoonfuls over mixture (some of the peanut butter cups will still be exposed).
4. Bake 25 to 30 minutes or until lightly browned. Cool completely in pan on wire rack. Cut into bars. About 16 bars.