HEAT oven to 350ºF.
MICROWAVE butter and chocolate squares in large microwaveable bowl on HIGH 1-1/2 min. or until butter is melted, stirring after 45 sec. Stir until chocolate is completely melted. Stir in sugar. Blend in eggs and vanilla. Add flour and cocoa powder; mix well.
SPREAD onto bottom of 9-inch square pan sprayed with cooking spray. Top with small spoonfuls of peanut butter; swirl gently with knife. Sprinkle with salt.
BAKE 18 to 20 min. or until toothpick inserted in center comes out with fudgy crumbs. (Do not overbake.) Cool completely.