Heat oven to 350 degrees. Spray a 9x13" pan with cooking spray. In large bowl,beat sugar,butter and peanut butter till light and fluffy. Add milk,vanila and egg;beat until well blended. Add flour baking soda and salt;mix well. spread in pan.
Bake 19 to 24 mins or till edges are golden brown. Immediately sprinkle with chocolate chips;let stand 5 mins.
Using metal spatula,spread softened chips to frost bars. Sprinkle with peanuts. Cool completely in pan on cooling rack,about 2 hours. For bars,cut in 6 rows by 6 rows.