Directions

• Birthday Cake:
• Heat the oven to 350°F.
• Combine the butter, shortening, and sugars in the bowl of a stand mixer fitted with the paddle attachment and cream together on medium-high for 2 – 3 minutes. Scrape down the sides of the bowl, add the eggs, and mix on medium-high for 2 – 3 minutes. Scrape down the sides of the bowl once more.
• On low speed, stream in the buttermilk, oil, and vanilla. increase the mixer speed to medium-high and paddle for 4 – 6 minutes, until the mixture is practically white, twice the size of your original fluffy butter-and-sugar mixture, and completely homogenous. Don’t rush the process. You are basically forcing too much liquid into an already fatty mixture that doesn’t want to make room for that liquid. there should be no streaks of fat or liquid. stop the mixer and scrape down the sides of the bowl.
• On very low speed, add the cake flour, baking powder, salt, and the ¼ cup rainbow sprinkles. Mix for 45 – 60 seconds, just until your batter comes together.  Scrape down the sides of the bowl.
• Pam-spray a quarter sheet pan and line it with parchment, or just line the pan with a silpat. using a spatula, spread the cake batter in an even layer in the pan. Sprinkle the remaining 2 tbsp rainbow sprinkles evenly on top of the batter.
• Bake the cake for 30 – 35 minutes. The cake will rise and puff, doubling in size, but will remain slightly buttery and dense.  At 30 minutes, gently poke the edge of the cake with your finger: the cake should bounce back slightly and the center should no longer be jiggly. Leave the cake in the oven for an extra 3 – 5 minutes if it doesn’t pass these tests.
• Take the cake out of the oven and cool on a wire rack. 
• Birthday Cake Soak:
• Whisk together the milk and vanilla in a small bowl.
• It is recommended to use Brown Patisse and clear McCormick brand of vanilla extract.  Neither is of any fancy caliber, but it is recommended because these specific vanilla extracts are the flavor that most people relate to in their baked goods. 
• Birthday Cake Frosting:
• Combine the butter, shortening, and cream cheese in the bowl of a stand mixer fitted with the paddle attachment and cream together on medium-high for 2 to 3 minutes, until the mixture is smooth and fluffy. scrape down the sides of the bowl.
• With the mixer on its lowest speed, stream in the glucose, corn syrup, and vanilla. crank the mixer up to medium-high and beat for 2 to 3 minutes, until the mixture is silky smooth and a glossy white. scrape down the sides of the bowl.
• Add the confectioners’ sugar, salt, baking powder, and citric acid and mix on low speed just to incorporate them into the batter. Crank the speed back up to medium-high and beat for 2 to 3 minutes, until you have a brilliant stark white, beautifully smooth frosting. It should look just like it came out of a plastic tub at the grocery store! Use the frosting immediately, or store it in an airtight container in the fridge for up to 1 week.
• Birthday Cake Crumb:
• Heat the oven to 300°F
• Combine the sugars, flour, baking powder, salt, and sprinkles in the bowl of a stand mixer fitted with the paddle attachment and mix on low speed until well combined.
• Add the oil and vanilla and paddle again to distribute. The wet ingredients will act as glue to help the dry ingredients form small clusters; continue paddling until that happens.
• Bake for 15 minutes at 300°F
• Let the crumbs cool completely before using in a recipe or scarfing by the handful. Stored in an airtight container, the crumbs will keep fresh for 1 week at room temperature or 1 month in the fridge or freezer.
• Cake Assembly:
• Put a piece of parchment or a silpat on the counter. Invert the cake onto it and peel off the parchment or silpat from the bottom of the cake. Use the cake ring to stamp out 2 circles from the cake. These are your top 2 cake layers. The remaining cake “scrap” will come together to make the bottom layer of the cake.
• Clean the cake ring and place it in the center of a sheet pan lined with clean parchment or a silpat. Use 1 strip of acetate to line the inside of the cake ring.
• Put the cake scraps together inside the ring and use the back of your hand to tamp the scraps together into a flat even layer.
• Dunk a pastry brush in the birthday cake soak and give the layer of cake a good, healthy bath of half of the soak.
• Use the back of a spoon to spread one-fifth of the frosting in an even layer over the cake.
• Sprinkle one-third of the birthday crumbs evenly over the frosting. Use the back of your hand to anchor them in place.
• Use the back of a spoon to spread a second fifth of the birthday cake frosting as evenly as possible over the crumbs.
• With your index finger, gently tuck the second strip of acetate between the cake ring and the top ¼ inch of the first strip of acetate, so that you have a clear ring of acetate 5 – 6 inches tall – high enough to support the height of the finished cake. Set a cake round on top of the frosting, and repeat the process for layer 1 (if 1 of your 2 cake rounds is jankier than the other, use it here in the middle and save the prettier one for the top).
• Nestle the remaining cake round into the frosting. Cover the top of the cake with the remaining frosting. Give it volume and swirls, or do as we do and opt for a perfectly flat top. Garnish the frosting with the remaining birthday crumbs.
• Transfer the sheet pan to the freezer and freeze for a minimum of 12 hours to set. The cake and filling. the cake will keep in the freezer for up to 2 weeks.
• At least 3 hours before you are ready to serve the cake, pull the sheet pan out of the freezer and, using your fingers and thumbs, pop the cake out of the cake ring. Gently peel off the acetate, and transfer the cake to a platter or cake stand. Let it defrost in the fridge for a minimum of 3 hours (wrapped well in plastic, the cake can be refrigerated for up to 5 days).
• Slice the cake into wedges and serve.