Preheat oven to 325 degrees.
Cream butter, gradually adding sugars, beating well at medium speed of electric mixer.
Sift flour and cocoa together.
Combine sour cream and baking soda.
Add flour mixture to creamed mixture alternately with sour cream.
Mix just until blended after each addition.
Stir in vanilla.
Beat egg whites at room temperature until stiff peaks form.
Fold into batter.
Spoon batter into a greased and floured 10 inch tube pan.
Bake for 1 hour and 15 minutes or until a toothpick inserted in center comes out clean.
Cool in pan for 10 minutes.
Remove cake to wire rack and cool completely.