Preheat the oven to 325°F and set an oven rack in the middle position. Spray a 10-in (12-cup) Bundt pan generously with nonstick cooking spray with flour.

Use an electronic beater to mix butter and sugar until well combined or pale yellow in color. Add in the first egg. Beat well after each addition of egg until creamy. Scrape down the sides and add in vanilla and fold in the flour into the mixture and mix well.

Separately, mix the egg whites until they are stiff (until the peak stands straight up when the beaters are lifted from the mixture).

Divide the batter into three portions. Stir the cocoa powder into one portion and mix well.

Transfer the plain batter into the greased baking pan. Shake it lightly to distribute evenly. Pour the cocoa batter in the middle of baking pan, and add the third portion of plain batter.

Lightly swirl the cocoa batter with a butter knife. I usually do the "S" shape with the butter knife a few times. Do not over mix.

Bake until golden brown and cooked, about 40 minutes. If the top turns too brown before the cake is set, cover the top with aluminum foil at 20 minutes mark to prevent the top from being too brown and crusty.

Remove it from the oven and let cool on the wire rack for 5 minutes. Remove from the pan, sliced and serve immediately.