1. Heat oven to 350°F. Grease three 8- or 9-inch round baking pans; line with wax paper.
2. Stir together cocoa and water in small bowl until smooth; set aside. Beat butter, shortening, sugar, salt and vanilla in large bowl until well blended. Add eggs; beat well. Stir 1-1/2 teaspoons baking soda into 1 cup buttermilk; add to butter mixture alternately with flour.
3. Measure 1-2/3 cups batter into small bowl; stir in remaining 1/8 teaspoon baking soda and 3 tablespoons buttermilk, orange peel and orange extract. Pour into one prepared pan. Stir cocoa mixture into batter; divide evenly between remaining two prepared pans.
3. Bake 25 to 30 minutes or until wooden pick inserted in center comes out clean. Cool 10 minutes; remove from pans to wire racks. Carefully peel off wax paper. Cool completely.
4. Place one chocolate layer on serving plate; spread with some of the ORANGE BUTTERCREAM FROSTING. Top with orange layer; spread with frosting. Top with remaining chocolate layer; frost top and sides of cake. 10 to 12 servings.
*To sour milk: Use 1 tablespoon white vinegar plus milk to equal 1 cup; use 1/2 teaspoon white vinegar plus milk to equal 3 tablespoons.
ORANGE BUTTERCREAM FROSTING
2/3 cup butter or margarine, softened
6 cups powdered sugar, divided
2 teaspoons freshly grated orange peel
1-1/2 teaspoons vanilla extract
4 to 6 tablespoons milk
Beat butter, 1 cup powdered sugar, orange peel and vanilla in large bowl until creamy. Add remaining powdered sugar alternately with milk, beating to spreading consistency. If necessary, add additional milk, 1/2 teaspoon at a time, to get desired spreading consistency. About 3 cups frosting.