Let’s break down this iconic cake creation:

1. Make the Birthday Cake Crumble:

• Preheat oven to 300°F (150°C). Line a baking sheet with parchment or Silpat.
• In the bowl of a stand mixer with the paddle attachment, combine all dry crumble ingredients (sugars, cake flour, baking powder, salt, 20g sprinkles). Mix on low until combined.
• Add grapeseed oil and clear vanilla extract. Mix on low until small clusters form. Don’t overmix into a paste.
• Spread crumble clusters onto the prepared baking sheet.
• Bake for 15 minutes. It should be slightly dry but not browned.
• Cool completely. Break up any large clusters. Store airtight.

2. Make the Cake Layer:

• Preheat oven to 350°F (175°C). Spray a quarter-sheet pan (approx. 10×15 inches) with cooking spray and line with parchment or Silpat.
• In a medium bowl, whisk together cake flour, baking powder, salt, and 50g (1/4 cup) rainbow sprinkles. Set aside.
• In the bowl of a stand mixer with the paddle attachment, combine butter, shortening, and both sugars. Beat on medium-high for 2-3 minutes until light and fluffy. Scrape bowl.
• Add eggs one at a time, beating well and scraping the bowl after each addition.
• Turn mixer to low. Stream in the buttermilk, oil, and clear vanilla extract.
• Increase speed to medium-high and beat for 4-6 minutes until the mixture is significantly lighter, fluffy, very white, and fully emulsified (no streaks). Scrape bowl.
• On low speed, add the flour mixture in thirds (or all at once as recipe states, but thirds is safer), mixing just until the batter comes together (30-60 seconds). Do not overmix.
• Spread batter evenly in the prepared sheet pan. Sprinkle with the remaining 25g (2 Tbsp) rainbow sprinkles.
• Bake for 30-35 minutes, or until a cake tester inserted into the center comes out clean.
• Cool completely on a wire rack. Once cool, wrap well in plastic wrap and chill or freeze briefly to make cutting easier.

3. Make the Frosting:

• In a medium bowl, sift together powdered sugar, salt, and baking powder. Set aside.
• In the bowl of a stand mixer with the paddle attachment, combine room temperature butter, shortening, and cream cheese. Beat on medium-high for 2-3 minutes until smooth and fluffy. Scrape bowl.
• Turn mixer to low. Stream in the glucose, corn syrup, and clear vanilla extract.
• Increase mixer speed to medium-high and beat for 2-3 minutes until smooth and glossy white. Scrape bowl.
• Turn mixer to low and gradually add the sifted sugar mixture and citric acid (or lemon juice).
• Once combined, turn mixer to medium-high and beat for another 2-3 minutes until very white, smooth, and fluffy. Use immediately or store airtight in the fridge (bring to room temp and re-whip before use).

4. Make the Soak:

• Whisk together the milk and clear vanilla extract in a small bowl or measuring cup.

5. Assemble the Cake (Using Cake Ring & Acetate – Recommended Method):

• Place the chilled cake sheet on a cutting board. Using a 6-inch cake ring (or round cutter), cut out 2 full cake rounds. Use the scraps to piece together a third 6-inch round (this will be the bottom layer).
• Place the 6-inch cake ring on your serving plate or cake board. Line the inside of the ring with a strip of acetate (cake collar).
• Place the pieced-together cake layer inside the ring at the bottom.
• Brush the cake layer generously with about 1/3 of the milk soak using a pastry brush.
• Spread a thin, even layer of frosting over the soaked cake (about 1/2 cup).
• Sprinkle about half of the birthday cake crumble evenly over the frosting layer. Gently press the crumble into the frosting.
• Place one of the clean-cut cake rounds on top of the crumble layer. Brush generously with 1/3 of the milk soak.
• Spread another even layer of frosting (about 1/2 cup) over the second cake layer.
• Sprinkle the remaining birthday cake crumble evenly over the frosting. Gently press it in.
• Place the final clean-cut cake round on top. Brush generously with the remaining milk soak.
• Spread a final thin layer of frosting evenly over the top cake layer, smoothing it with an offset spatula right up to the acetate edge.
• Chill the assembled cake (still in the ring with acetate) thoroughly in the refrigerator for at least 4-6 hours, or preferably overnight, until very firm.

6. Finish and Serve:

• Once the cake is fully chilled and firm, carefully remove the cake ring. Gently peel off the acetate strip.
• The cake is traditionally served with the exposed sides showing the layers. You can garnish the top edge with extra crumble if desired.
• Let the cake sit at room temperature for 15-20 minutes before slicing with a hot, sharp knife for clean cuts.

This assembly method is key to the signature Momofuku look!