Chiffon cakes are mostly leavened by the meringue that is folded into the batter, making them light and fluffy in texture. This recipe has a mild chocolate flavor from the cocoa powder used to create the red velvet marbling technique and is topped with a traditional cream cheese frosting.
For the marbling: In a small bowl, whisk together the cocoa powder, sugar, vegetable oil, and food coloring. Whisk in the boiling water until the mixture is smooth and set aside to cool to room temperature.
For the chiffon cake: Sift the cake flour with 3/4 cup of the sugar into a separate medium bowl. Add the baking powder and salt and whisk together. Set aside.
In a separate medium bowl, whisk together the 7 egg yolks, milk, vegetable oil and vanilla until combined.
Add the wet mixture to the dry ingredients and whisk together to form a loose batter. Be sure to continue whisking until all the clumps of flour have been completely incorporated and the batter is smooth. Set aside
Meanwhile, add the 9 egg whites to the bowl of a stand mixer fitted with a whisk attachment and begin to beat on medium speed until foamy. Add the cream of tartar and continue beating. Begin slowly streaming in the remaining 3/4 cup sugar and continue beating until the meringue holds stiff, glossy peaks, 2 to 3 minutes.
With a rubber spatula, stir a third of the meringue into your batter, folding it in to preserve as much air in the egg whites as possible. Gently fold in the remaining meringue and continue to stir until completely incorporated.
Remove 2 cups of the batter and fold into your cooled chocolate mixture.
Grease two 9-inch round cake pans with baking spray, butter or shortening. Cut 2 parchment circles and line the bottoms of the pans. Grease the paper as well.
Divide half of the vanilla cake batter evenly between the prepared cake pans. Dot the chocolate batter on top. Cover with the remaining vanilla batter. With a toothpick, skewer or butter knife, swirl the batters around to create a marble effect.
Bake until a cake tester comes out clean, 25 to 30 minutes. Remove from the oven and allow the cakes to cool in the pans for 10 minutes. Gently pull the sides away from the edges of the pan with a butter knife and invert cakes onto a cooling rack to cool completely. Remove the parchment.
For the cream cheese frosting: Add the cream cheese to the bowl of a stand mixer fitted with the paddle attachment and mix until smooth. Add the butter and continue beating on medium until smooth.
Add half the confectioners' sugar and mix starting on low and increasing to medium-high speed until fluffy. Add the remaining confectioners' sugar and the vanilla extract. Mix until fluffy, smooth and velvety. If the mixture is too thick, loosen with a splash of milk. Add a bit more confectioners' sugar if the frosting seems too loose. Transfer the frosting to a pastry bag fitted with a large star tip.
Place a cooled cake layer on a serving platter with the domed side down. Pipe about 1 1/2 cups cream cheese frosting into large rosettes on top and spread evenly with an offset spatula. Top with the second cake layer, domed-side up. Frost the top and sides of the cake with the remaining cream cheese frosting.
Make the chocolate mixture first so it can cool to room temperature. Grease the pans very well as the cake is very sticky.