1Heat oven to 350ºF. Grease square pan, 9x9x2 inches. Mix sugars, butter, milk and eggs in medium bowl. Stir in flour, baking powder and salt.
2Divide batter in half (about 1 cup plus 2 tablespoons for each half). Stir peanut butter and peanut butter chips into 1 half. Stir cocoa and chocolate chips into remaining half.
3Spoon chocolate batter into pan in 8 mounds, checkerboard style. Spoon peanut butter batter between mounds of chocolate batter. Gently swirl through batters with knife for marbled design.
4Bake 30 to 35 minutes or until toothpick inserted in center comes out clean. Cool completely about 1 hour. For 16 brownies, cut into 4 rows by 4 rows.