1. Preheat oven to 350 degrees Fahrenheit.
2. In the large bowl of an electric mixer, beat the egg whites until foamy. Gradually add 3/4 cup sugar in a steady stream. Beat until stiff peaks form. Set aside.
3. In another bowl, beat the remaining sugar with the egg yolks until light. Gradually add oil, juices and vanilla sugar and beat well. Combine flour and baking powder. Add to batter and beat well until batter lightens in color.
4. Fold yolk mixture into whites with a rubber spatula, blending well.
5. Separate 3/4 cup of batter. Place all the ingredients for the chocolate batter into a bowl. Mix well. Add the 3/4 cup batter and mix well again.
6. Pour the white batter into an ungreased 10-inch tube pan. Gently pour the chocolate mixture onto the top of the white mixture in drizzling motion. Swirl gently with fork to mix — about six to eight strokes.
7. Bake for one hour.
8. When cake is fully baked, remove from oven and turn pan upside down immediately. Cool completely. Run a sharp knife around the edges of pan before removing cake.