First, prep the chocolate chip sheet cake for cutting. Place a piece of parchment paper large enough to hold the chocolate chip cake on your counter. Invert the cake onto it; peel and discard the parchment from the bottom of the cake.
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.
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.
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).
Make the bottom layer of the Milk Bar chocolate chip cake. 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!
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 passion fruit puree for assembly. Use a pastry brush to brush half of the passion fruit puree for assembly over the layer. If using a digital scale to weigh out the passion fruit puree, you'll need approximately 1 ounce (30 grams) of passion fruit puree for this layer.
Spread the half of the passion fruit curd over the bottom layer. Use the back of a spoon or a small rubber spatula to spread half of the passion fruit curd in an even layer over the cake. If using a digital scale to weigh out the passion fruit puree, you'll need approximately 6.25 ounces (177 grams) of passion fruit curd for this layer. Don't worry too much about making the top of the passion fruit curd 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. Sprinkle half of the chocolate crumbs over the passion fruit curd on the bottom layer. Sprinkle half of the chocolate crumbs over the passion fruit curd. If using a digital scale to weigh out the chocolate crumbs, you'll need approximately 3 ounces (85 grams) of chocolate 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! Spread a third of the coffee frosting over the chocolate crumbs on the bottom layer. Use the back of a spoon or a small rubber spatula to spread ⅓ of the coffee frosting as evenly as possible over the chocolate crumbs. If using a digital scale to weigh out the coffee frosting, you'll need approximately 1.15 ounces (33 grams) of coffee frosting for this layer.This is the most difficult process of assembling the layer cake! The coffee frosting will pull some of the crumbs up, and refuse to spread over their rough texture. But similar to the previous step of spreading the passion fruit curd, don't worry about making the frosting smooth. 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. Make the middle layer of the Milk Bar chocolate chip cake. 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 frosting. 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 passion fruit puree, the remaining half of the passion fruit curd, the remaining half of the chocolate crumbs, and a ⅓ (1.15 ounces or 33 grams) of the coffee frosting.
Make the top layer of the Milk Bar chocolate chip cake. Once you've finished assembling the passion fruit puree, passion fruit curd, chocolate crumb, and coffee layers of the coffee frosting, set the final cake round on top of the frosting.
Frost the top layer of the Milk Bar chocolate chip cake with the remaining coffee frosting. Use the back of a spoon or a small rubber spatula to spread the remaining coffee frosting as evenly as possible over the top layer. Again, don't worry about making the frosting smooth! You'll cover it with mini chocolate chips in the next step.
Garnish the frosted Milk Bar chocolate chip cake with the mini chocolate chips for assembly. Garnish the frosting with the remaining mini chocolate chips for assembly.
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.
Thaw the cake, then serve. At least 3 hours before you are ready to serve the cake, pull the sheet pan out of the freezer and use your fingers and thumbs to gently pop the cake out of the cake ring. Transfer the cake to a serving plate or cake stand. Gently peel off the acetate. Defrost in the fridge for a minimum of 3 hours (or 1 hour at room temperature) before serving.
Store the cake. The assembled cake will keep, uncovered in the refrigerator, for up to 1 day. After that, tightly wrap in plastic wrap and refrigerate for an additional 3 days. Leftover slices can be stored in an airtight container in the fridge.