1. Heat oven to 350 F. Line 15-1/2x10-1/2x1-inch jelly-roll pan with foil; grease foil.
2. Melt butter in medium saucepan over low heat. Add cocoa; stir constantly until smooth. Remove from heat; stir in sugar and vanilla. Beat in eggs, one at a time, until well combined. Stir in flour and baking powder. Spread batter evenly in prepared pan.
3. Bake 14 to 16 minutes or until top springs back when touched lightly in center. Remove from oven; cool 2 minutes. Invert onto wire rack. Peel off foil; turn right side up on wire rack to cool completely.
4. Prepare PEANUT BUTTER FILLING AND GLAZE. Cut brownie in half; spread half of glaze evenly on one half. Top with second half; spread with remaining glaze. Cool until glaze is set. Prepare CHOCOLATE DRIZZLE; drizzle over glaze. After chocolate is set, cut into bars. About 40 bars.
PEANUT BUTTER FILLING AND GLAZE: Combine 1/3 cup sugar and 1/3 cup water in small saucepan; cook over medium heat to boiling. Remove from heat; immediately add 1-2/3 cups (10-oz. pkg.) REESE'S Peanut Butter Chips. Stir until melted. Cool slightly. About 1-1/3 cups glaze.
CHOCOLATE DRIZZLE: Place 1/3 cup HERSHEY'S SPECIAL DARK Chocolate Chips or HERSHEY'S Semi-Sweet Chocolate Chips and 1 teaspoon shortening (do not use butter, margarine, spread or oil) in small microwave-safe bowl. Microwave at MEDIUM (50%) 30 seconds to 1 minute or until chips are melted and mixture is smooth when stirred.