Preheat oven to 325°. Generously grease and flour a tube pan. Set aside.
In a medium bowl, mix flour, salt and baking soda. Set aside. In a small bowl, mix sour cream and buttermilk until smooth. Set aside. Keep egg whites in a medium-large bowl. Cover the bowl with plastic to make sure nothing splashes in it.
In large mixing bowl, cream butter and sugar until smooth.
Add eggs and vanilla. Beat until well combined.
Add 1/3 c. of the flour mixture. Mix just until combined. Then add 1/2 of the sour cream-buttermilk blend. Mix just until combined. Add the rest of the flour mixture alternately with the wet mixture and mix until combined. Let it sit while you beat the egg whites.
Make sure the beaters are completely clean or the egg whites won't beat. When they are ready, they will be slightly stiff and should form peaks that curl over when you lift the beaters.
Carefully fold the egg whites into the batter until all combined.
Spoon into the tube pan. Lift pan a few inches and drop onto counter to release any air bubbles.
Bake for 1 hour. Depending on where you live and your oven, baking time may vary. A toothpick inserted in center will come out clean.
Let it cool in the pan for about 10 minutes. Remove from pan and place on serving platter or cake saver.
Glaze while still warm.
Recipe for glaze:
3 tablespoons real butter
1 cup powdered sugar
3 tablespoons milk
1 teaspoon vanilla.
Melt butter in small sauce pan. Remove from heat. Stir in sugar, milk and vanilla until smooth. Pour over warm cake.