Preheat oven to 325 degrees F (165 degrees C). Grease an flour one 10 inch tube pan.
To make marbling mixture: Combine 1/3 cup of the coca and 1/2 cup of the 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 of the sugar and the brown sugar. Beat in the egg yolks, then the vanilla and sour cream.
In another bowl, stir together the flour, cocoa, salt, and baking soda. Beat into the creamed mixture. Stir about 1/3 of the egg whites into the batter to lighten it and then 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 (175 degrees C) for 1 3/4 hours, or until it tests done with a toothpick. Let cool on a rack. Makes 16 to 20 servings.