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Preheat oven to 350°F. For the Cake, mix cornstarch, sorghum flour, tapioca flour, xanthan gum, baking powder, baking soda and salt in medium bowl. Set aside.
Beat granulated sugar, butter, sour cream and vanilla in large bowl with electric mixer on medium speed 5 minutes or until light and fluffy. Beat in eggs, 1 at a time. Gradually add flour mixture alternately with milk, beating on low speed after each addition until smooth. Do not overbeat. Pour batter evenly into 2 (9-inch) round cake pans sprayed with no stick cooking spray.
Bake 30 to 35 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.