Preheat oven to 350 F. For the Cake, grease 2 (9-inch) round cake pans with oil. Coat with each pan with 1 teaspoon of the cocoa powder. Set aside. Mix remaining 1 cup cocoa powder, cornstarch, sorghum flour, tapioca flour, xanthan gum, baking powder, baking soda and salt in medium bowl. Set aside.
Beat granulated sugar and oil in large bowl with electric mixer on medium speed until well blended. Add melted chocolate; mix well. Beat in eggs, 1 at a time. Stir food color and vanilla into buttermilk. Gradually beat in flour mixture alternately with buttermilk mixture on low speed until just blended. Do not overbeat. Pour batter into prepared pans.
Bake 40 to 45 minutes or until toothpick inserted in center comes out clean. Cool in pan 10 minutes. Remove from pans; cool completely on wire rack.