Preheat oven to 350°F. Grease and line four 8" pans or four 7" pans with parchment rounds and 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 sour cream in two installments, on a low speed.

Add in 2 tsp of vanilla extract and mix at a low speed until incorporated.

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

If you want to color the layers in an ombre fashion, add a small squirt of either pink or blue gel food coloring in the bowl and stir just until evenly colored. Scoop 440 grams of batter into one of the pans. Repeat 3 more times, creating increasingly deeper colored cake batter as you add in more gel food coloring.

Bake for 32-35 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 before you plan to assemble your cake. Use a 2 inch circle cutter to remove the center from the two middle cake layers, leaving the darkest and lightest cake layers full. If you're making these in advance, wrap and freeze them at this point.

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 buttercream frosting.

Beat 2 cups of unsalted butter on a medium-high speed for 1-2 minutes with a paddle attachment, until smooth. It should become slightly lighter in color.

Mix in 1 tbsp of vanilla extract and 1 tsp salt on a low speed.

Slowly mix in 7 cups of powdered sugar on a low speed. Half way through add in 1/3 cup of heavy cream to make the frosting easier to mix.

Continue to mix on low speed for a few minutes, until 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).

Cover the frosting with plastic wrap to prevent crusting and set aside.

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 cake layer and work your way up to the lightest.

Add an even layer of buttercream between each cake layer with a large offset spatula.

Once you've stacked your third cake layer, you should have a nice little cavity in the center of your cake. Fill the opening with a matching sprinkle blend. Top with the lightest, uncut cake layer.

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

Place 1/4 of the buttercream into a small bowl and set aside. Divide the remaining buttercream into two bowls. Use gel food coloring to color one pink and one blue.

Spread the pink frosting on the bottom half of the cake, followed by white buttercream in the middle. Cover the top of the cake with blue frosting on the top half of the cake. Blend the colors together with a small offset spatula, then smooth using a bench scraper, then chill the cake.

Decorate as desired! I chose to top my cake with a white chocolate ganache drip, some pink and blue macarons, frosting swirls and some matching candy.