Cream butter and sugar together until light and fluffy.
Add egg yolks, one at a time, beating well after each addition.
Sift flour with soda; add to but- ter mixture alternately with sour cream and vanilla.
Blend until smooth.
Fold in stiffly beaten egg whites; pour into greased and floured tube pan.
Bake at 300 degrees for 1 hour and 30 minutes or until cake tests done.
Let cool for 10 minutes; remove from pan and cool thoroughly.