• Vanilla Cake Layers
• Preheat the oven to 350 F / 175 C. Grease and line 5, 7-inch or 5, 8-inch cake pans (I used 7-inch pans). If you don't have 5 pans, don't worry! Grease & line however many you have and bake the cake layers in a couple rounds.
• Add 1 cup of unsalted butter into the bowl of a stand mixer or a large bowl and beat with a whisk attachment on a medium speed until smooth and creamy.
• Add 3 cups of granulated sugar and mix together on a high speed for 2-3 minutes with a whisk attachment or hand mixer. Scrape the sides of the bowl with a rubber spatula as needed. The mixture should become lighter in color.
• Pour in 1 cup of egg whites and mix at a medium-low speed until incorporated.
• In a separate, medium-sized bowl, whisk together 3 cups cake flour, 2 1/2 tsp baking powder and 1 tsp salt. Add half of the dry ingredients into the butter/egg mixture and mix on a medium speed until combined.
• Next, add 1 1/2 cups of sour cream, 1/4 cup vegetable oil, and 1 tsp vanilla extract into the batter. Mix at a medium speed until combined, scraping the sides and bottom of the bowl as needed.
• Mix in the remaining half of the dry ingredients on a medium speed.

We need to make 2 blue layers, 2 pink layers, and 1 white layer. There are two ways to do this:

• Option 1: Divide the batter evenly between five bowls (about 360g per bowl). Color two of the bowls pink, and two of them blue with gel food coloring. Leave one uncolored.
• Option 2: If you have a kitchen scale, fill two, separate bowls with 720g of batter each. Leave the remaining batter in the mixing bowl uncolored. Color one the bowls with 720g of batter a light pink, and the other bowl light blue.
• Next, pour the batter into the prepared pans. If you don't have enough pans, the extra batter can sit at room temperature. Bake the layers that you can, let them cool, then clean out the pans and use them to bake the remaining layers.
• If you make 7-inch cake layers, bake them for 33-35 minutes, or until a toothpick comes out with a few moist crumbs. Or if you make 8-inch cake layers, bake them for 29-32 minutes. Rotate the pans halfway through to help them bake evenly.
• Let the pans cool for 10 minutes, then run an offset spatula around the perimeter of the pan to separate the cake from the pan.
• Place the 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 top of each cake layer. I also trimmed the caramelization from the sides of the cake, but this is optional. Set the cake layers aside or wrap them and freeze them at this point if you're making them in advance.

Vanilla Buttercream Frosting

• While the cake layers bake and cool, make the vanilla buttercream.
• Beat 2 1/2 cups of butter on a medium speed for 30 seconds with a paddle attachment or in a large bowl with a hand mixer until smooth. 
• Add 1 tsp of vanilla extract and 1/2 tsp fine salt. Mix on low until the ingredients are fully incorporated.
• Slowly mix in 8 cups of powdered sugar. Halfway through, add in 1/3 cup of heavy cream to make the frosting easier to mix.
• Once all the powdered sugar has been incorporated, mix on a low speed until the ingredients are fully incorporated and the desired consistency is reached. I also like to stir it by hand with a rubber spatula once it's fully made. This will help make the frosting silky smooth, and easier to smooth onto your cake.
• Wait to color the frosting until the cake has been crumb coated. Cover the buttercream flush with plastic wrap and set aside.

Assembling This Trans Flag Cake

• Stack and frost the cake layers on a greaseproof cake board or flat plate, using a dab of vanilla buttercream to help stick the first cake layer to the board.
• Start with one of the blue layers, and spread a thick, even layer of buttercream on top of it. Repeat this process with the remaining cake layers in the following order: pink, white, pink, blue.
• Once you've stacked the cake layers, smooth a thin coat of vanilla buttercream around the cake with a large offset spatula and bench scraper. Freeze the cake for about 10 minutes in the freezer or 30 minutes in the fridge, or until the frosting is firm to the touch.
• Place 1/4 of the uncolored, remaining frosting in a small piping bag fit with a small round tip or cut a small, 1 cm opening at the base of the bag. Set aside.
• Add 1/3 of the remaining frosting in a medium-sized bowl and color a light shade of pink with gel food coloring (I used a small drop of deep pink gel food coloring).
• Color the remaining buttercream light blue with gel food coloring (I used a small drop of royal blue gel food coloring). You want to have slightly more blue frosting than the other colors, because it is also used to cover the top of the cake. Place the pink and blue frosting in small piping bags fit with a small round tip (like a Wilton 10) or cut a small, 1 cm opening at the base of each bag.
• Pipe the frosting in lines around the cake to match the cake layers inside. You want to make 5 bands that are about the same thickness. Start with a band of blue frosting, followed by pink, white, pink, then blue. Cover the top of the cake with blue buttercream, then smooth the frosting with a large offset spatula and bench scraper.
• If desired, pipe a message on the side of the cake. Then add the remaining buttercream inside a small piping bag fit with a small French tip (like a Wilton 4B). Pipe a border around the top of the cake, add some sprinkles, then enjoy!