Lightly coat the bottom of a 9x9x2-inch baking pan with nonstick cooking spray, being careful not to coat sides of pan.
In a medium saucepan melt butter; remove from heat. Stir in granulated sugar, the water, and vanilla. Stir in flour, cocoa powder, and baking powder until combined. Stir in chocolate pieces. Pour batter into prepared pan.
Bake in a 350 degree F oven for 15 to 18 minutes or until a wooden toothpick inserted near the center comes out clean. Cool on a wire rack. Remove from pan. Cut into 16 bars. Sprinkle with the powdered sugar. Makes 16 brownies.