Preheat oven to 350°F. and butter and flour a 9-inch square baking pan, knocking out excess flour. In a 1 1/2-quart bowl stir together topping ingredients until combined well. In a 1 1/2-quart saucepan melt butter over moderately low heat. Remove pan from heat and stir in sugar and vanilla. Add eggs, 1 at a time, beating well with a wooden spoon after each addition. Into a bowl sift together flour, cocoa powder, baking powder, and salt and stir into butter mixture until just combined well. Spread batter evenly in pan and sprinkle with topping. Bake brownies in middle of oven 30 to 35 minutes, or until a tester comes out with crumbs adhering to it. Cool brownies completely in pan on a rack before cutting into 16 squares. Brownies keep, layered between sheets of wax paper in an airtight container at cool room temperature, 5 days.