1. Preheat oven to 350 degrees F.
2. Coat the bottom of a 9-x-9-inch baking pan with nonstick spray. In a medium saucepan, melt butter; remove from heat. Stir in sugar, water, and vanilla. Mix in flour, cocoa powder, and baking powder until combined. Add chocolate pieces. Pour batter into pan; bake for 15 to 18 minutes. Cool in pan on wire rack. Sprinkle with powdered sugar.