Let’s walk through this detailed process:

Make the Buttermilk Cake Layers:

1. Preheat & Prep Pans: Preheat oven to 350°F (180°C). Grease bottoms of three 8×2-inch round cake pans, line with parchment rounds, and grease the parchment.
2. Combine Dry Ingredients & Butter (Reverse Creaming): In the bowl of an electric mixer with the paddle attachment, whisk together the cake flour, superfine sugar, baking powder, and salt. With the mixer on low speed, add the cold butter pieces one at a time. Beat until all butter is incorporated and the mixture resembles fine cornmeal, about 3 minutes.
3. Combine Wet Ingredients: In a medium measuring cup with a spout, combine 1 cup of the room temperature buttermilk, the vanilla, and the almond extract. In a separate measuring cup, gently whisk the room temperature eggs with the remaining 1/2 cup buttermilk.
4. Add Liquids (Part 1): Increase mixer speed to medium-low. Gradually add the vanilla/buttermilk mixture (from the first cup) to the dry ingredients. Beat for 5 minutes. Scrape the bowl well.
5. Add Liquids (Part 2): Reduce mixer speed to low. Gradually add the egg/buttermilk mixture in three additions, scraping the bowl after each. Beat until well combined, about 2 minutes total.
6. Final Fold & Bake: Fold the batter once or twice with a spatula to ensure everything is incorporated. Divide batter evenly among the prepared pans. Bake in the center of the oven for 20 to 22 minutes, or until a wooden pick inserted into the center comes out with a few moist crumbs.
7. Cool: Let cake layers cool in pans on wire racks for 10 minutes. Loosen sides with a knife and carefully turn cakes out onto racks. Peel off parchment and let cool completely. Wrap cooled layers tightly in plastic wrap; can be stored at room temp for up to 3 days.

Make the Birthday Cake Frosting:

1. Cream Butter & Shortening: Combine room temperature butter and shortening in the stand mixer bowl (paddle attachment). Beat on medium-high for about 3 minutes until creamy and fluffy. Scrape bowl.
2. Add Liquid Sugars & Extracts: With mixer on low, stream in the corn syrup, vanilla extract, and lemon extract (or citric acid). Turn mixer to medium-high and beat for 3 minutes until smooth and glossy white. Scrape bowl.
3. Add Dry Ingredients: Add the sifted confectioners’ sugar and salt. Beat on low speed until incorporated.
4. Whip Frosting: Turn mixer back up to medium-high and beat for 3 more minutes until shiny, creamy, and smooth.
5. Add Cream Cheese: Add the softened cream cheese and beat on medium speed just until combined, about 1 minute. Do not over-beat at this stage.

Assemble the Buttermilk Birthday Cake:

1. Prepare Frosting & Sprinkles: Reserve about 2 cups of the white frosting for tinting/piping. Gently stir the 1/2 cup of sprinkles into the remaining white frosting (the portion you’ll use for filling and the final coat).
2. Level Cakes: Trim the cooled cake layers if needed for flat surfaces.
3. First Layer: Place one cake layer, top up, on a cake board or plate (reserve the best-looking layer for the top). Using a small offset palette knife, spread 1 1/4 cups of the sprinkle-filled frosting evenly over the top, extending slightly past the edge.
4. Repeat Layers: Add the second cake layer and repeat the frosting/sprinkle application. Place the final cake layer on top, bottom side up (for a flat top surface).
5. Crumb Coat: Using a straight palette knife, smooth the frosting protruding from between the layers onto the sides of the cake. Apply a thin layer of the sprinkle frosting over the entire cake to seal in crumbs. Chill for 15 minutes.
6. Final Frosting Coat: Remove cake from the fridge. Apply a smooth, thick final layer of the sprinkle-filled frosting over the chilled crumb coat.
7. Tint and Pipe Border (Optional): Tint the reserved 2 cups of frosting with a few drops of turquoise gel color. Fit an 18″ pastry bag with a large closed star tip (Ateco #887). Fill the bag with the turquoise frosting and pipe a decorative border around the top perimeter of the cake.
8. Serve/Store: The cake will keep at room temperature for up to 12 hours. Refrigerate for longer storage (up to 3 days), but serve at room temperature if possible for best texture.

This assembly creates the signature look with sprinkles in the frosting.