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First, prep the birthday cake for cutting. Use the overhanging parchment as handles to lift the brownies out of the pan and onto a cutting board.
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Cut the sheet cake into the different layers. Use the cake ring to stamp out 2 full circles and 2 half circles from the cake.
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The full circles will be your top two cake layers, while the half circles and remaining cake scraps will come together to make the bottom layer of the cake.
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Prep the cake ring and acetate cake collar. Clean the cake ring and place it in the center of a sheet pan lined with clean parchment paper. Line the inside of the cake ring with the acetate paper (if using two strips of acetate paper, just use one strip).
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Make the bottom layer. Nestle the two half circles in the cake ring, pressing their round sides against the cake ring. There will be a gap in the middle of the circles—this is totally okay, I promise!
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Fill the gap between the two half circles with the remaining cake scraps. Use the back of your hand to press the scraps together to fit into the space. Head's up that you're going to have more scraps than can fit in the gap—that's totally normal, I promise! Simply take the excess scraps and press them down evenly on top of the rest of the cake layer, using the back of your hand to create a flat and even layer.At this point, the acetate collar will start to wrinkle from the pressure of the cake. Don't worry about it too much! It will even back out when you add the middle and top layer. -
Brush the bottom layer with half of the Birthday Cake Soak. Use a pastry brush to brush half of the Birthday Cake Soak for assembly over the layer. If using a digital scale to weigh out the soak, you'll need approximately 1 ounce (30 grams) of Birthday Cake Soak for this layer. -
Spread a third of the Birthday Cake Frosting over the soaked cake. Use the back of a spoon or a small rubber spatula to spread ⅓ of the Birthday Cake Frosting as evenly as possible over the soaked cake If using a digital scale to weigh out the frosting, you'll need approximately 5 ounces (142 grams) of frosting for this layer.
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Don't worry too much about making the top of the frosting smooth—you'll cover it with other ingredients in the next step. The more important thing is to try and spread it as evenly as you can across the top of the cake, and ensure that too much of it doesn't get up on the sides of the acetate collar.
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Sprinkle a third of the Birthday Cake Crumbs over the frosted cake. Sprinkle a third of the Birthday Cake Crumbs over the frosted cake. If using a digital scale to weigh out the crumbs, you'll need approximately 3 ounces (85 grams) of crumbs for this layer. -
Use the back of your hand to gently press the crumbs in place. If you want to go the extra mile, sprinkle slightly more crumbs along the outer edges of the cake. Christina Tosi recommends doing so in a YouTube tutorial of hers—doing so creates more contrast between layers in the final, assembled cake!
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Make the middle layer. If you're using two acetate cake collars, gently tuck the second strip of acetate between the cake ring and the top ¼ inch of the first strip of acetate. Doing so creates a ring of acetate that is 5 to 6 inches tall, combined—this will be high enough to support the height of the finished cake.Set a cake round on top of the Birthday Cake Crumbs. If one of your cake rounds is jankier than the other, use it in the middle and save the pretty one for the top! -
Repeat the process for brushing the middle layer with the remaining half of the Milk Bar soak. Use a pastry brush to brush the remaining half of the Birthday Cake Soak over this cake.
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Repeat the process for frosting the soaked cake with a third of the Birthday Cake Frosting. Use the back of a spoon or a small rubber spatula to spread ⅓ of the Birthday Cake Frosting as evenly as possible over the soaked cake.If using a digital scale to weigh out the frosting, you'll need approximately 5 ounces (142 grams) of frosting for this layer. -
Repeat the process for sprinkling a third of the Birthday Cake Crumbs over the frosted cake. Sprinkle a third of the Birthday Cake Crumbs over the frosted cake. If using a digital scale to weigh out the crumbs, you'll need approximately 3 ounces (85 grams) of crumbs for this layer. -
Make the top layer. Set the final cake round on top of the Birthday Cake Crumbs.
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Repeat the process for frosting the cake with the remaining Birthday Cake Frosting. Use the back of a spoon or a small rubber spatula to spread the remaining ⅓ of the Birthday Cake Frosting as evenly as possible over the cake.If using a digital scale to weigh out the frosting, you'll need approximately 5 ounces (142 grams) of frosting for this layer. -
Repeat the process for sprinkling the remaining third of the Birthday Cake Crumbs over the frosted cake. Sprinkle a third of the Birthday Cake Crumbs over the frosted cake. If using a digital scale to weigh out the crumbs, you'll need approximately 3 ounces (85 grams) of crumbs for this layer.I like to leave a little circle of frosting in the center of the cake for "aesthetic"—however, you can cover the frosting completely if you'd like! Freeze the cake. Transfer the sheet pan with the cake to the freezer. Freeze, uncovered and fully assembled, for a minimum of 12 hours to set the cake and all its fillings. The cake will keep in the freezer for up to 2 weeks.