Preheat oven to 350°F. Line 13x9-inch baking pan with foil, with ends of foil extending over sides of pan. Spray foil with no stick cooking spray. Mix 1 cup of the cornstarch, sorghum flour, tapioca flour, baking powder, xanthan gum and salt in medium bowl. Set aside.
Microwave butter in large microwavable bowl on HIGH 1 1/2 minutes or until butter is melted. Stir in brown sugar until well blended. Add eggs and vanilla; mix well. Gradually stir in flour mixture until well blended. Toss chocolate chips with remaining 2 tablespoons cornstarch. Add to batter; mix well. Spread in prepared pan.
Bake 30 to 35 minutes or until toothpick inserted in center comes out clean. Cool in pan on wire rack. Use foil handles to remove dessert from pan. Place on cutting board and cut into bars.