Step 1

Preheat the oven to 300 degrees F. Line a rimmed sheet pan (10x15 jelly roll pan) with parchment paper or Silpat.

Step 2

Combine the sugars, flour, baking powder, salt and sprinkles in the bowl of a mixer fitted with a paddle attachment. Mix on low speed until well combined. Add the oil and vanilla and mix on low speed until small clusters form.

Step 3

Spread the clusters on the prepared sheet pan. Bake for 20 minutes, breaking up the clusters once or twice during baking. The crumbs should be slightly moist to the touch; they will dry and harden as they cool. Allow the crumbs to cool completely on the pan before using. The birthday cake crumbs can be stored in an airtight container at room temperature for 1 week, or in the refrigerator or freezer for up to 1 month.

Step 4

Preheat oven to 350 degrees F. Spray a quarter sheet pan with non-stick cooking spray and line with parchment paper, or with Silpat.

Step 5

In a measuring cup, combine the buttermilk, grapeseed oil, and vanilla extract.

Step 6

In a medium bowl, whisk together the flour, baking powder, salt, and ¼ cup of the sprinkles.

Step 7

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 to 3 minutes. Scrape down the sides of the bowl, add the eggs, and mix on medium-high for 2 to 3 minutes. Scrape down the sides of the bowl once more.

Step 8

On low speed, gradually pour in the buttermilk mixture. Increase the mixer speed to medium-high and beat for 4 to 6 minutes, until the mixture has nearly doubled in size and is completely homogenous (don't rush this step!). Stop the mixer and scrape down the sides of the bowl.

Step 9

On low speed, gradually add the flour mixture and mix just until the batter comes together, about 45 to 60 seconds. Scrape down the sides of the bowl and give the batter a final mix with a rubber spatula.

Step 10

Spread the cake batter in an even layer in the prepared pan. Sprinkle the remaining 2 tablespoons rainbow sprinkles evenly on top of the batter. Bake the cake for 30 to 35 minutes; when you gently poke the edge of the cake with your finger, the cake should bounce back slightly, and the center should no longer be jiggly. Place the pan on a wire rack to cool completely. The cooled cake can be stored in the refrigerator, wrapped in plastic wrap, for up to 5 days.

Step 11

In a small bowl, whisk together the glucose, corn syrup and vanilla extract.

Step 12

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.

Step 13

With the mixer on its lowest speed, stream in the glucose mixture. Increase the mixer to medium-high speed and beat for 2 to 3 minutes, until the mixture is silky smooth and glossy. Scrape down the sides of the bowl.

Step 14

Add the powdered sugar, salt, baking powder and citric acid, and mix on low speed just to incorporate them into the batter. Increase the speed to medium-high and beat for 2 to 3 minutes, until the frosting is completely smooth. Use the frosting immediately, or store in an airtight container in the refrigerator for up to 1 week. (Bring to room temperature before using in the recipe.)

Step 15

Put a piece of parchment or 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.

Step 16

Layer 1, The Bottom: 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.

Step 17

Put the cake scraps in the ring and use the back of your hand to tamp the scraps together into a flat even layer.

Step 18

In a small bowl, whisk together the milk and vanilla extract for the birthday cake soak. Using a pastry brush, brush half of the soak over the top of the cake.

Step 19

Using the back of a spoon, spread one-fifth of the frosting in an even layer over the cake.

Step 20

Sprinkle one-third of the birthday cake crumbs evenly over the top of the frosting. use the back of your hand to press them gently into the frosting.

Step 21

Use the back of a spoon to spread a second fifth of the frosting as evenly as possible over the crumbs.

Step 22

Layer 2, The Middle: With your index finger, gently tuck the second strip of acetate between the cake ring and the top 1/4 inch of the first strip of acetate, so that you have a clear ring of acetate 5 to 6 inches tall. Set one of the cake rounds (the more imperfect of the two) on top of the frosting, brush with the remaining soak, and repeat the process for Layer 1 (a fifth of frosting, a third of crumbs, and another fifth of frosting).

Step 23

Layer 3, The Top: Nestle the remaining cake round into the frosting. Cover the top of the cake with the last fifth of the frosting. Garnish the frosting with the remaining birthday crumbs.

Step 24

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.)

Step 25

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 refrigerator for a minimum of 3 hours before slicing and serving. (You can store the cake, wrapped well in plastic wrap, in the refrigerator for up to 5 days.)