Preheat oven to 350°F. Mix sugar and cocoa powder in large bowl. Set aside. Mix cornstarch, sorghum flour, tapioca flour, xanthan gum, baking powder and salt in medium bowl. Set aside.
Microwave butter in large microwavable bowl on HIGH 2 minutes or until melted. Stir in cocoa mixture until well blended. Add eggs and vanilla; mix well. Gradually stir in flour mixture until well blended. Spread in foil-lined 13x9-inch baking pan sprayed with no stick cooking spray.
Bake 30 to 35 minutes or until toothpick inserted in center comes out clean. Cool in pan on wire rack. Cut into bars.