Beat butter at medium speed with an electric mixer (not a hand held one) about 2 minutes or until creamy. Gradually add sugar, beating at medium speed 5 to 7 minutes. Combine cake flour and baking soda, and add to butter mixture 1 cup at a time. (Batter will be extremely thick).
Separate eggs; add yolks to batter, and mix well. Stir in sour cream and vanilla. Beat egg whites until stiff, and fold into batter.
Spoon into a greased and floured 12-cup Bundt or 10-inch tube pan. Bake at 300F for 2 hours or until a wooden pick inserted in the center comes out clean. (You may also spoon batter into two 9- x 5- x 3-inch loaf pans; bake at 300F for 1 1/2 hours or until a wooden pick inserted in center comes out clean). Cool in pan on wire rack 10 to 15 minutes. Remove from pan; cool completely on wire rack.
NOTE: Standard Mixing Method: If you are using a handheld mixer or prefer a conventional pound cake method, here's the procedure we suggest.
Beat butter at medium speed with an electric mixer about 2 minutes or until creamy. Gradually add 3 cups sugar, beating at medium speed 5 to 7 minutes. Add eggs, one at a time, beating just until yellow disappears.
Combine cake flour and baking soda; add to butter mixture alternately with sour cream, beginning and ending with flour mixture. Mix at lowest speed just until mixture is blended after each addition. Stir in vanilla. Bake as directed.