Preheat oven to 325. Combine sifted flour with soda and salt. Sift and set aside.
Cream butter and sugar thoroughly with an electric mixer. Add egg yolks and beat until fairly smooth. Add flour mixture in batches, alternating with sour cream, blending by hand with a rubber spatula or whisk. Blend in vanilla or mace.
Beat egg whites until they hold a stiff peak but are not dry. Gently fold into cake batter.
Spoon batter into a greased and floured heavy 10" tube pan or two 8 x 4 x 3 loaf pans, filling pans not more than 3/4 full. Bake until cake springs back at once when lightly touched (about an hour and 15 minutes--will vary with pans used). Cake tester should come out clean. Remove cake from oven and allow it to rest 5 minutes.
Run a thin knife around the edges of the pan to loosen cake and turn out onto a rack. Turn the cake right side up to cool. Store in airtight box. Serves 10-15.
