preheat oven to 350°. Brush an 8 x 8" square baking pan with butter. Line pan with a piece of parchment paper, leaving a 2 inch overhang on sides; grease paper.
In a large bowl whisk butter and sugars until smooth. Whisk in egg and vanilla. Add flour and salt; mix just until moistened. Do not over mix. Fold in 1/2 cup each chocolate chips and candy coated peanut butter pieces. Transfer batter to prepared pan. Smooth top with spatula. Sprinkle with remaining chocolate chips and peanut butter pieces.
Bake 40 to 45 minutes or until top is golden brown and a toothpick inserted into the center comes out clean. Set pan on wire rack and let cool completely. Using parchment overhang, lift brownies from pan and transfer to a cutting board. Makes 16 bars.