Heat oven to 350 degrees. Remove wrappers from candies. Chop candies into pieces. Greas 13x9x2-inch baking pan. In small mixer bowl, beat butter, peanut butter, sugar and brown sugar until well blendedd. Gradually add milk, eggs, and vanilla, beating until smooth and well blended. Stir together flour, baking soda and salt. Add to butter mixture; beat until well blended. Spread batter into prepared pan. Bake 18 to 20 minutes or until wooden pick inserted in center comes out clean. Cool completely in pan on wire rack. Spread Chocolate Peanut Butter Topping over cake. Sprinkle candy pieces over top. Cut into squares.
Chocolate Peanut Butter Topping: In medium microwave-safe bowl, place chocolate chips, peanut butter and butter. Microwave at HIGH 1 minute; stir. If necessary, microwave at HIGH an additional 15 seconds at a time, stirring after each addition, just until chips are melted when stirred. Add powdered sugar; whisk until smooth and of spreading consistency.