Congrats, you made it to the assembly portion of the cake! This is where you'll need the cake ring and the acetate sheets. Place a large sheet of parchment paper on a work surface. Invert the Yellow Custard Cake onto it and use the cake ring to stamp out 2 circles from the cake. These are your top 2 cake layers. The remaining cake "scraps" will come together to make the bottom layer of the cake — you should still have enough cake leftover to stamp out two half circles. You'll use these half circles for the bottom layer of the cake, and fill the gap between the half circles with the leftover cake scraps. Clean the cake ring and place it on the center of a half sheet pan lined with parchment paper. Line the inside of the cake ring with two sheets of acetate, creating a weird plastic tube that extends upwards beyond the cake ring. Carefully nestle the two cake layer halves into the cake ring, filling the gap between the halves with extra cake scraps to form a complete layer. Don't be afraid to use the back of your hand to tamp the scraps together to create a flat, even layer. Dunk a pastry brush in the Cake Soak and give the layer of cake a good, healthy bath of about 1/3 of the soak. Use the pastry bag of Blackberry Swiss Meringue Buttercream and pipe an even layer of around 1/3 of the frosting across the cake, using the back of a spoon to smooth and even out the frosting as necessary.
Sprinkle one-third of the Birthday Cake Streusel evenly over the top of the frosting. Use the back of your hand to anchor them in place.
Carefully nestle a second layer of cake on top of the Birthday Cake Streusel. Repeat steps 3, 4 and 5 to soak the layer with Cake Soak and top it with another layer of Blackberry Swiss Meringue Buttercream and Birthday Cake Streusel.
Carefully nestle the third and final layer of cake on top of the Birthday Cake Streusel. Repeat steps 3, 4, and 5 to soak the layer with the last of the Cake Soak and top it with the last of the Blackberry Swiss Meringue Buttercream and Birthday Cake Struesel.
Transfer the sheet pan containing the cake 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.