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

Mix together 3 cups all purpose flour, 3 cups sugar, 2 1/2 tsp baking powder and 1 tsp salt 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 your prepared pans. I like to use a digital scale to make sure each pan has the same amount of batter, so that each cake layer is the same height.

Bake for 32-34 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.

If you make these cake layers in advance and freeze them, let them thaw for about 20 minutes before making your cake. The cake layers should still be slightly cold to the touch, which will make it easier to assemble your cake.

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

Beat 2 cups of unsalted butter on a medium speed for 30 seconds until smooth with a paddle attachment / stand mixer or a hand mixer.

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

Slowly add in 7 cups of powdered sugar, 1 cup at a time. Alternate with 3 Tbsp of heavy cream or milk, and mix on a low speed. I like to place a kitchen towel over my mixer to contain any powdered sugar clouds.

Mix on low 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 1/3 of the frosting in a separate bowl and color it black with 3 squirts of black gel food coloring, 1/3 cup black cocoa, 1/2 cup of melted and cooled dark chocolate and 1 additional Tbsp heavy cream. Mix with a rubber spatula until the frosting is evenly colored.

Leave the remaining frosting uncolored. Cover both frosting bowls with plastic wrap and set aside.

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

Add an even layer of the uncolored white vanilla buttercream between each cake layer with a large offset spatula.

Add a thin coat of frosting around the cake, fully cover the cake layers.

Smooth using a bench scraper, then chill the cake in the fridge (20 minutes) or freezer (5 minutes) until the frosting is firm to the touch.

Add a second, thicker layer of black frosting around the sides of the cake and smooth using a bench scraper.

Spread a thin layer of white frosting on top of the cake.

Color 1/2 cup of frosting a salmon color with pink and orange gel food coloring, and 1/2 cup green with electric green food coloring.

Use a small offset spatula to cover a 4 inch circle on top of the cake with the green and salmon colored frosting, making half the circle salmon colored and the other half green. The goal is to make this look like the salmon and avocado filling in a sushi roll. Pipe thin lines of of white frosting on top of the orange frosting to look like thin ribbons of fat that are found in salmon.

Fill in the top of the cake around the green and orange frosting with white jelly beans to look like rice.