Preheat oven to 350°F. Line four 7" or three 8" pans with parchment rounds, and grease with non-stick baking spray.

Mix together 2 1/2 cups all purpose flour, 3 cups sugar, 1 1/2 tsp baking powder, 1 tsp salt and 2 tsp of instant coffee (optional) in a stand mixer with a paddle attachment or hand mixer until fully combined.

Mix in 1 cup of unsalted butter slowly into the dry ingredients on a low speed. Continue to mix until no large chunks of butter remain and the mixture looks crumbly.

Pour in 1 cup of egg whites and mix on low until just incorporated. Mix in 1 1/2 cups buttermilk in two installments, on a low speed.

Add in 2 tsp of vanilla extract and 2 Tbsp of vegetable oil, and mix at a low speed until fully incorporated.

Scrape down the sides of the bowl with a rubber spatula, then beat on a low speed for about 30 seconds to make sure everything is properly mixed together.

Divide the batter evenly between three or four bowls (depending on how many cake layers you're making). I like to use a digital kitchen scale to weigh my pans, and ensure they all have the same amount of batter.

Bowl 1: 3 Tbsp all-purpose flour (24 grams) and 1 1/2 tsp dark cocoa powder (3 grams)

Bowl 2: 2 Tbsp all-purpose flour (16 grams) and 1 Tbsp dark cocoa powder (6 grams)

Bowl 3: 1 Tbsp all-purpose flour (8 grams) and 2 Tbsp dark cocoa powder (12 grams)

Bowl 4: 3 Tbsp dark cocoa powder (18 grams)

Pour each color of batter into one of the prepared cake pans.

Bake for 34-35 minutes, or until a toothpick comes out with a few moist crumbs. Allow the pans to cool for 10 minutes, then run a small offset spatula around perimeter of the pan to separate the cake from the pan.

Place cake layers into the freezer for 30 minutes to accelerate the cooling process. Once the layers are fully cooled, carefully flip the pans and remove the layers.

Use a serrated knife to level the tops of the layers right before you plan to assemble your cake, or you can wrap and freeze them in you're making them in advance.

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

Beat 3 cups of unsalted butter on a medium speed for 30 seconds with a paddle attachment, until smooth.

Mix in 1 Tbsp of vanilla extract or vanilla bean paste and 1 tsp salt on a low speed.

Slowly add in 10 cups of powdered sugar, 1 cup at a time. Alternate with small splashes of 1/3 cup of cream.

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

Divide the frosting evenly between three or four bowls (depending on how many cake layers you're making).

Bowl 1: 1 tsp regular unsweetened cocoa powder (6 grams)

Bowl 2: 2 Tbsp dark cocoa powder (12 grams)

Bowl 3: 3 Tbsp dark cocoa powder (18 grams)

Bowl 4: 3 Tbsp dark cocoa powder (18 grams) and 1/4 cup melted dark chocolate (40 grams)

NOTE: If any of the shades of frosting seem too thick, add in additional cream (1 TBSP at a time). If the frosting seems too thin / not pipeable, add in more powdered sugar (quarter of a cup at a time). You can learn more about frosting consistency (and what to look for) here.

Stack and frost cake layers on a greaseproof cake board, using a dab of frosting to help stick the first cake layer to the board.

Start with the darkest chocolate cake layer, and frost it with an even layer of the most chocolatey buttercream. Repeat with the remaining cake layers, moving from the most intensely chocolate to the least.

Add a thin coat of frosting around the the cake using the overhanging frosting to fully cover the cake layers.

Smooth using a bench scraper, then chill the cake in the fridge (20 minutes) or freezer (10 minutes) until the frosting is firm to the touch.

Place the remaining frosting in large piping bags fit with a Wilton 1M frosting tip, and pipe buttercream roses around the cake.