Preheat the oven to 350 F, and position a rack in the center of the oven. Grease the bottoms of three 8-inch cake pans with nonstick baking spray.
In a small bowl, whisk together the cocoa powder and hot water until smooth. Set aside.
In a large bowl, sift together the flour, sugar, baking powder and salt, then whisk until well combined.
In a separate bowl, whisk together the eggs, egg whites, buttermilk, melted butter, oil and vanilla. Add the dry ingredients to the wet, and whisk just until combined, about 10-15 seconds.
Remove 3/4 cup of the vanilla cake batter, and stir in the cocoa powder mixture to make a chocolate batter, so that you have a large bowl of vanilla cake batter and a small bowl of chocolate cake batter.
Spoon about 1/2 cup of the vanilla cake batter into each of the cake pans. Add about 2 tablespoons of the chocolate batter right in the center of the vanilla batter. Repeat with another 1/2 cup of vanilla cake batter in the center of the chocolate batter - it will make the chocolate batter spread out into a ring. Repeat with another 2 tablespoons of chocolate batter. Continue alternating vanilla and chocolate batter in each of the pans, until you've used up all the batter. Now use a toothpick to gently and briefly swirl the two batters together; you don't want to blend the batter too much.
Bake the cakes for about 21-25 minutes, until the tops spring back when gently touched and a toothpick comes out clean.
Set the pans on a cooling rack, cover loosely with a clean kitchen towel, and cool completely.