Heat oven to 350 F. Grease 8- or 9-inch baking pan.
Place butter in medium microwave-safe bowl. Microwave at HIGH (100%) 1 to 1-1/2 minutes or until melted. Stir in sugar and vanilla. Add eggs, one at a time, beating well with spoon after each addition. Add cocoa; beat until well blended. Add flour, baking powder and salt; beat well. Set aside 2 tablespoons baking pieces; stir remaining pieces into batter. Spread batter in prepared pan.
Bake 30 to 35 minutes or until brownies begin to pull away from sides of pan. Cool completely in pan on wire rack.
Place remaining 2 tablespoons baking pieces in small microwave-safe bowl. Microwave at HIGH 15 seconds; stir. If necessary, microwave an additional 15 seconds at a time, stirring after each heating, until chocolate is melted and smooth when stirred. Drizzle over brownie surface. Refrigerate until drizzle sets up. Cut into bars.