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Preheat oven to 350°F. Line four 8-inch round cake pans with parchment rounds, and grease with non-stick baking spray.
Begin by making the vanilla cake batter. Mix together all dry ingredients (flour, baking powder, sugar, and salt) in a stand mixer with a paddle until fully combined.
Mix chunks of room-temperature butter slowly into the dry mix, on a low speed. Continue to mix until no large chunks of butter remain, and the mixture becomes crumbly.
Pour in egg whites, and mix on low until just incorporated. Mix in the buttermilk in two installments, on a low speed. Add in vanilla and oil, and mix at a low speed until fully incorporated. Scrape down the sides of the bowl with a spatula, then beat on medium speed for about 30 seconds.
Pour half of the batter into a separate bowl (roughly 950 grams), and set aside.
Next, convert the vanilla batter into chocolate cake batter. Mix the additional 1/3 cup baking cocoa, 1/4 sugar, 1/4 tsp salt, and 1/2 tsp baking powder into the remaining batter.
Add the additional 1/4 cup egg whites, and 1/4 cup buttermilk, and mix on a low speed until smooth.
If desired, make the chocolate fudge swirl. Combine the chocolate chips, butter, and heavy cream into a heatproof bowl. Heat in two 30-second intervals, stirring in between. Allow the mixture to sit for 1 minute, then stir until smooth and set aside to cool.
Alternate spoonfuls of chocolate and vanilla cake batter into the prepared cake pans, filling them roughly one inch high with batter (I like to use a digital kitchen scale to make sure each pan has the same amount of batter).
Add small drops (about one inch in diameter) of the chocolate swirl on top of the cake batter, then drag a small offset spatula or butter knife through the batter in horizontal and vertical lines to create a marbled pattern.
Bake for 33-35 minutes (or until a skewer comes out clean). Allow to cool for 10 minutes, then run an offset spatula around perimeter of the pan to separate the cake from the pan.
If desired, level the cakes with a serrated knife once they're fully cooled.
Beat the butter on a medium speed for 30 seconds with a paddle attachment, until smooth.
Mix in the vanilla extract and salt on a low speed.
Slowly add in the powdered sugar, 1 cup at a time. Alternate with small splashes of cream.
Mix on low for a couple minutes until the ingredients are fully incorporated, and the desired consistency is reached.
If the frosting is too thick, add in additional cream (1 Tbsp at a time). If the frosting is too thin, add in more powdered sugar (quarter of a cup at a time).
Place half of the buttercream into a seperate bowl. Mix in 1/2 cup of baking cocoa, and the melted chocolate chips. This will be used to fill the cake, and will be the darkest buttercream of the squiggles. Place in a large piping bag.
With the remaining vanilla buttercream, place half of the into another large piping bag.
Color the rest of the frosting with the other 1/2 cup of baking cocoa, to create a light brown colored frosting. Place in a large piping bag.
Once all the different components of this cake are ready, begin to assemble the cake!
Stack the cake layers on a greaseproof cardboard cake round.
Spread a thin dab of buttercream on the center of the board, to help your cake layer stay in place.
Next, center the first cake layer the cake board.
Spread an even layer of the dark chocolate buttercream onto the cake layer with a large offset spatula. Repeat with remaining cake layers.
Place the top cake layer upside down.
Cover the cake in a thin layer of frosting, and smooth using a bench scraper.
Chill the cake in the freezer for about 5 minutes, until the frosting is firm to the touch.
Pipe squiggly lines of buttercream around the cake, alternating between the vanilla, chocolate, and dark chocolate buttercream.