Remove from heat and stir in sugar, eggs, vanilla, peanut butter, flour, and baking powder until well blended.
Spread batter evenly in a buttered and floured 8-inch square baking pan.
Bake in a 350 oven until edges feel firm to touch and begin to turn a shade darker, 25 to 30 minutes.
Run a knife between pan rim and brownie.
Let cool in pan on a rack, about 1 hour.
Cut into 9 or 16 squares.
Chocolate glaze. In a 1- to 2-quart pan over low heat, frequently stir 1/3 cup whipping cream and 1 cup (6 oz.) semisweet chocolate chips until melted and smooth. Stir in 1 teaspoon vanilla. Use warm.
