First, preheat your oven to 350 degrees. Then, grease and flour 2 - 8 inch round cake pans.
Next, beat the egg whites until soft peaks form. Add 2 Tablespoons of sugar to the egg whites and beat on "high" power. Add another 2 tablespoons of sugar and then beat again until it is fully incorporated. Set aside.
In a large bowl on low speed, beat the remaining sugar with flour, buttermilk, vegetable oil, baking powder, salt, and vanilla extract.
Divide the mixture into 2 separate bowls. In one bowl, beat in half of the egg whites.
In the other bowl, beat in the cocoa powder and 2 Tablespoons of granulated sugar on low speed. Then, beat in the other half of the egg whites.
Spoon half of the vanilla cake batter into each of the pans. (Leaving spaces between sections of batter in the cake pans, because this is where we will be putting the chocolate cake batter.) Then spoon half of the chocolate batter into each pan in the empty spaces. (See photos in blog post for example.)
Drag a butter knife through the batter to swirl the batter for the marble effect.
Bake for 25 minutes and let cool on wire rack.
Place the first layer on a plate or cake stand and cover with chocolate frosting, then top with the second layer of cake and finish by frosting the top and edges of the cake.
Notes
How to store leftover cakeThis cake is ok to sit out at room temperature. However, if you do frost it with the chocolate buttercream recipe we recommend, it would be best to refrigerate it, as it contains milk and butter.Store in an airtight container for 3-4 days.