Preheat oven to 350º. Butter and lightly flour three 9 inch round pans. Sift SWANS DOWN Cake Flour, then lightly spoon into measuring cup 3 cups. Add baking soda and salt to cake flour, set aside. In a very large bowl, cream butter; gradually add sugar, beating until well blended. Add egg yolks, one at a time, blending into batter after each addition. Add vanilla extract, mix until just blended. Alternately add in flour mixture and buttermilk, blending well with each addition. In another bowl, with clean beaters, beat egg whites from the 4 eggs until stiff peaks form (approx. 6-7 minutes). Fold the beaten egg whites gently into cake batter until well mixed. Bake approx. 20 minutes or until sides of cake pull away from pan and tester inserted in center of cake comes out clean. Cool in pan for 10-15 minutes on a wire rack; remove from pan and let cool completely on wire rack before frosting.
Note: ***This is a large cake, yields approx. 8 cups of batter. Make sure mixing bowl is large enough to hold this batter. ***Frosted cake approx. 3¾ inches in height, make sure cake plate/cover large enough to keep the cake air tight. ***Buttermilk Substitution: When do not have buttermilk, substitute: 1 tablespoon vinegar or lemon juice plus enough milk to make one cup. Let stand 5 minutes before using.