Preheat oven to 350°F (180°C). Spray an 8-inch (20 cm) square baking pan with cooking spray. In a medium bowl, using a fork, stir flour with cocoa, salt, and baking powder until no cocoa lumps remain. In a medium saucepan, melt butter over medium heat. Remove from heat and stir in sugar then egg, egg whites, and vanilla until combined. Stir in flour mixture just until blended.