Beat butter and shortening at medium speed with an electric mixer until creamy; gradually add sugar, beating until blended. Add eggs, 1 at a time, beating just until blended.
Combine cake flour and next 3 ingredients; add to butter mixture alternately with buttermilk, beginning and ending with flour mixture. Beat at low speed until blended after each addition. Stir in vanilla.
Pour 7 cups batter into a greased and floured 13- x 9-inch pan, reserving remaining 1 cup batter.
Bake at 350°F for 35 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack 15 minutes; remove from pan, and cool completely on wire rack.
Note: Prepare cake batter 1 1/2 more times; stir in reserved 1 cup batter. Repeat procedure for baking the second 13- x 9-inch layer. Spoon 3 cups batter into a greased and floured 9- x 5-inch pan. Bake at 350°F for 35 to 40 minutes or until a wooden pick inserted in center comes out clean. Repeat procedure with remaining batter for baking the second 9- x 5-inch layer.
Yield 1 (13- x 9-inch) layer (8 cups batter).