Heat oven to 350 degrees F. Line an 8-inch square baking dish with foil so it hangs over sides; coat foil with nonstick cooking spray and set aside.
In a small bowl, blend flour, cocoa, baking powder and salt.
In a medium-size microwave-safe bowl, microwave chocolate and butter for 40 seconds until melted, stirring halfway through. Stir until smooth, heating in 15 second increments if needed. Cool 2 minutes, then whisk in sour cream, chocolate syrup, vanilla, egg, egg white and sugar. Stir in flour mixture.
Spread batter into prepared pan and bake at 350 degrees F. for 25 to 28 minutes or until a toothpick inserted in the center comes out clean. Remove brownie from pan using foil handles. Cut into squares and top each with 1 scoop frozen yogurt, drizzle with chocolate sauce and finish with chopped nuts and cherry, if desired. Makes 9 servings.