Preheat oven to 350°F. Line three eight-inch pans or four seven-inch 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 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. 

Add in a generous squirt of blue gel food coloring. Mix the batter on a low speed, scraping the sides and bottom of the bowl part way through to evenly color the batter.

Divide batter evenly between the prepared cake pans. I like to use a digital kitchen scale to weigh my pans, and ensure they all have the same amount of batter. This guarantees your layers will bake to be the same height.

Bake for 34-36 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.

Place cake layers into the freezer for 45 minutes, to accelerate the cooling process.

Once the cake layers are cooled. level the tops with a serrated knife.

Use a large circle cutter to remove the center from two of the cake layers, to allow you to fill the cake with sprinkles.

Beat the butter on a medium speed for 30 seconds with a paddle attachment, until smooth. Slowly add in the powdered sugar, 1 cup at a time. Alternate with small splashes of cream.

Once fully mixed, add in the vanilla and salt, and beat 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).

Wait to color the buttercream blue with gel food coloring until you've filled and crumb coated the cake. The white buttercream will have a great contrast against the blue cake layers.

Heat heavy cream in a heatproof bowl in the microwave for 45 seconds, until it's just beginning to bubble. Gently pour white chocolate chips into the heavy cream, making sure they are fully covered with cream. Allow mixture to sit for 1 minute.

Stir slowly until the cream and white chocolate are combined. Some bits of chocolate may not be fully melted yet. Heat the mixture again for 20 seconds, and stir. Repeat as necessary until all the chocolate bits are fully melted and incorporated.

Add in the blue gel food coloring, if desired. Allow to cool until desired viscosity is reached (barely warm to the touch).

If it seems too thin or thick, you can add a touch more chocolate chips or heavy cream! The type of cream and white chocolate that you use can affect the consistency, so feel free to adjust as needed.

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

Add an even layer of buttercream between each cake layer.

Add the two cut cake layers next.

Frost the inside cavity of the cake with a thin layer of buttercream. Fill with blue and white sprinkles and sixlets, and a few pieces of gelt (if desired).

Top with the final uncut cake layer.

Spread 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.

Color the remaining frosting different shades of blue, leaving one cup uncolored. Place each color into a piping bag and cut openings varying in size from 1/2 cm to 1 cm.

Pipe stripes of different colored frosting around the cake, and smooth using a bench scraper.

Add the blue drips to the cake using a spoon.

Pipe swirls on top of the cake with a french tip, and place pieces of gelt on top.