Step 1

Preheat oven to 350°F / 175 C. Grease and line four 8" cake pans and set aside.

Step 2

Begin by making the vanilla cake batter. Mix together the dry ingredients (3 cups flour, 3 cups sugar, 2 1/2 tsp baking powder, and 1 tsp salt) in a stand mixer with a paddle until fully combined.

Step 3

Mix 1 cup of room-temperature butter into the dry ingredients on a low speed. Continue to mix until no large chunks of butter remain, and the mixture looks like moist sand.

Step 4

Pour in 1 cup of egg whites and mix on low until just incorporated, then mix in 1 1/2 cups of buttermilk, 2 Tbsp vegetable oil and 2 tsp vanilla extract. on a low speed.

Step 5

Scrape down the sides of the bowl with a spatula, then beat on medium speed for about 1 minute. This helps give the layers a lighter texture.

Step 6

Pour 2/3 of the batter into a separate bowl (roughly 1200 grams) and set aside.

Step 7

In the other bowl, transform the vanilla cake batter into chocolate cake batter. Mix in the additional 1/3 cup baking cocoa, 1/4 sugar, 1/4 cup egg whites, 1/4 cup buttermilk, 1/2 tsp baking powder, and 1/4 tsp salt on a low speed until combined and set aside.

Step 8

If desired, make the chocolate fudge swirl. Combine 1 cup of chocolate chips, 1/4 cup of room temperature butter, and 1/4 cup heavy cream into a heatproof bowl. Heat in two 30-second intervals, stirring between each interval. Allow the mixture to sit for 1 minute, then stir until smooth.

Step 9

Alternate spoonfuls of chocolate and vanilla cake batter into the prepared cake pans. Fill them roughly 1-inch high with batter. I like to use a digital kitchen scale to make sure each pan has the same amount of batter.

Step 10

Add small drops (about 1-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. Save the leftover chocolate mixture to decorate the cake layer. It can sit out at room temperature for up to 8 hours.

Step 11

Bake for 32-35 minutes or until a toothpick comes out with a few moist crumbs.

Step 12

Once the cake layers are out of the oven, run a small offset spatula around the edge of the pans to help separate the cake layer from the pan.

Step 13

Allow the layers to cool in the pan for about 15 minutes, then flip them onto a wire rack to finish cooling.

Step 14

Once the cake layers have finished cooling use a serrated knife to level the tops and set the cake layers aside.

Step 15

If you're making these cake layers in advance, wrap and freeze them like this. When you're ready to assemble the cake, unwrap the frozen layers and allow them to thaw for about 30 minutes before you plan to frost the cake.

Step 16

While the cake layers bake and cool, make the vanilla buttercream frosting.

Step 17

Beat 2 cups of unsalted butter on a medium speed for 30 seconds with a paddle attachment until smooth.

Step 18

Mix in 1 Tbsp of vanilla extract and 1/2 tsp salt on a low speed.

Step 19

Slowly add in 7 cups of powdered sugar on a low speed. Add 1/4 cup of heavy cream halfway through to make the frosting easier to mix.

Step 20

Continue to mix on low speed for a few minutes, until the desired consistency is reached.

Step 21

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).

Step 22

Place 1 cup of vanilla buttercream into a separate bowl and set aside.

Step 23

Mix in 1/2 cup of baking cocoa, 1 cup of melted chocolate chips and an additional 1/4 cup of heavy cream in to the remaining frosting.

Step 24

Cover both the vanilla and the chocolate buttercream with plastic wrap to prevent crusting and set aside.

Step 25

Stack the cake layers on a greaseproof cardboard cake round.

Step 26

Spread a tablespoon of buttercream on the center of the board to help your cake layer stay in place.

Step 27

Center the first cake layer on 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.

Step 28

Cover the cake in a thin layer of frosting and smooth using a bench scraper.

Step 29

Chill the cake in the freezer for 10 minutes or fridge for 30 minutes until the frosting is firm to the touch.

Step 30

Swirl together the vanilla and chocolate buttercream around the cake using the back of a spoon or an offset spatula. Heat up the remaining chocolate fudge swirl mixture for 15-30 seconds, or until it's thin enough to easily spread into the frosting on the cake. Swirl the chocolate mixture into the frosting to create a marbled look, then enjoy!