Preheat oven to 350°F. Line four 7 inch pans round pans with parchment rounds, and grease with non-stick cooking spray.

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 extract and oil, and mix at a low speed until fully incorporated.

Scrape down the sides of the bowl with a spatula, then beat on low speed for about 15 seconds (the key is to mix until the ingredients are JUST incorporated, so that the batter isn't over-mixed). 

Fold the sprinkles into the cake batter using a rubber spatula, just they've evenly distributed.

Divide batter evenly between the prepared cake pans (about 475 grams per pan).

Bake for 34-37 minutes, or until a toothpick comes out clean. Allow the pans to cool for 10 minutes, then run an offset spatula around perimeter of the pan to separate the cake from the pan.

Place cake layers into the freezer for 45 minutes, to accelerate the cooling process. Once the layers are fully cooled, carefully flip the pans and remove the layers from the pans.

Use a serrated knife to level the tops of the layers.

Remove the centers from two of the cake layers, using a 3 inch circle cutter. This will create a chamber for the sprinkles once the layers are stacked.

Beat the butter on a medium speed for 30 seconds with a paddle attachment, until smooth. 

Add in the sifted black cocoa, and mix on low until the cocoa is fully incorporated, and no clumps remain.

Slowly add in the powdered sugar, 1 cup at a time. Alternate with small splashes of cream.

Mix in the cooled, melted chocolate.

Once fully mixed, add in the black gel food coloring, salt and vanilla.

Beat on low until the ingredients are fully incorporated, and the desired color and consistency is reached.

If the frosting is too thick, add an extra Tbsp of heavy cream. If it's too thin, add in another 1/2 cup of powdered sugar.

Stir by hand with a rubber spatula to get the frosting SUPER smooth. Place into large piping bags, to make assembling and decorating the cake easier.

Build the cake on a greaseproof cake board, using a dab of frosting to help stick the first cake layer to the board. Begin with one of the full cake layers

Add an even layer of buttercream on top of the cake layer.

Stack and frost the two cake layers that have had their centers removed.

Pour rainbow jimmies into the center of the cake, to fully fill the space.

Top of the remaining uncut cake layer.

Apply a thin coat of frosting around the the cake, to fully cover the cake layers. Smooth using a bench scraper, then chill the cake in the fridge (10 minutes) or freezer (5 minutes) until the frosting is firm to the touch.

Add a second, thin layer of frosting around the center of the cake. Gently press rainbow sprinkles into the frosting with your hands.

Pipe a thick boarder around the sprinkles, and the fill in above and below the piping with additional frosting. Smooth using both an offset spatula and a bench scraper.

Chill the cake for another 5 minutes in the fridge (or 20 in the freezer), and then paint the edges of the fault line and top lip of the cake with edible gold paint.