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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 vanilla into buttermilk. Gradually add flour mixture alternately with buttermilk mixture, beating on low speed after each addition until smooth. 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.
For the Frosting, beat butter and vanilla in large bowl until light and fluffy. Mix confectioners' sugar and cocoa powder. Gradually add to butter, beating well after each addition and scraping sides and bottom of bowl frequently. Add milk; beat until light and fluffy. If frosting is too thick to spread, gradually beat in additional milk. Fill and frost cooled cake with Frosting.