Preheat oven to 325 degrees F. Grease and flour one 10 inch tube pan.

To make marbling mixture: combine 1/3 cup of the cocoa and 1/2 cup white sugar and mix well.

In a large bowl, beat the egg whites until stiff, and set aside. In another bowl, cream the butter with 1 cup sugar and brown sugar. Beat in the egg yolks, then vanilla and sour cream.

In another bowl, stir together the flour, cocoa, salt and baking soda. Beat into creamed mixture. Stir about 1/3 of the egg whites into batter to lighten it and fold in the rest gently but thoroughly. Spread about 1/4 of the batter into a greased and floured tube pan and sprinkle with about 1/3 of the cocoa-sugar mixture. Continue repeating layers, ending with the batter.

With a knife, lightly swirl the batter and cocoa mixture together. Bake at 325 degrees F for 1-3/4 hours, or until it tests done with a toothpick. Let cool on a rack.