Make the crumble: Preheat oven to 300F and line a baking sheet with parchment paper or a silpat.
In the bowl of a stand mixer fitted with the paddle attachment, combine all dry ingredients. Mix on low until well combined.
Add oil and vanilla, mix until small clusters form. Bake for 15-25 minutes. Let cool and either store in an airtight container or freeze.
Make the cake: Preheat oven to 350F and spray a half-sheet pan (11”x 16”) with cooking spray and line with parchment paper or a silpat. OR spray and line three 6" cake pans.
In a medium bowl, whisk together cake flour, baking powder, salt, and 50g of sprinkles.
In the bowl of a stand mixer fitted with the paddle attachment, combine butter, shortening, and sugars. Beat on med-high for 2-3 minutes.
Add eggs one at a time, fully incorporating after each addition.
Turn mixer to low and stream in buttermilk, oil, and vanilla. Increase mixer to med-high and beat for 4-6minutes until mixture is light, fluffy, and no streaks of fat or liquid remain.
On low speed, add flour mixture and mix just until combined. Spread cake batter in an even layer in the pan and sprinkle with remaining sprinkles.
Bake for 30-35 minutes or until a toothpick comes out clean.
Make the frosting: In a medium bowl, sift together powdered sugar, salt, and baking powder.
In the bowl of a stand mixer fitted with the paddle attachment, combine butter, shortening, and cream cheese. Beat on med-high for 2-3 minutes until the mixture is smooth and fluffy.
Turn mixer to low and slowly add glucose, corn syrup, and vanilla. Increase mixer to med-high and beat for 2-3 minutes until smooth and glossy white.
Turn mixer to low and add in sugar mixture and citric acid. Once combined, turn mixer to med-high and beat until white and smooth.
Use immediately or store it in the fridge.
Make the soak: Whisk together the milk and vanilla in a small bowl or measuring cup.
Assemble the cake: If you baked the cake in a baking sheet, place a piece of parchment or silpat onto the counter and invert cake onto it. Peel off parchment/silpat from bottom of the cake.
Using the cake ring, stamp out 2 circles which will be your top and bottom layers. Stamp out 2 half circles from the remaining scraps.
If you baked the cake in 6" cake pans, simple remove the layers from the pan and continue with the rest of the directions.
Clean cake ring and place in the center of your quarter-sheet pan lined with parchment or a silpat.
Place 1 strip of acetate inside the cake ring to line it. **Make sure the cake ring is completely dry or you won’t be able to get it out of the ring after freezing!!**
Place your less pretty full layer into the bottom. Dip a pastry brush into the soak and brush onto the cake layer.
Using the back of a spoon, spread ⅕ of the frosting in an even layer over the cake.
Sprinkle ⅓ of the crumble evenly over the frosting. Press down with the back of your hand to anchor them in place. (I reserved the chunkiest crumbs for the very top).
Using the back of a spoon, spread ⅕ of the frosting in an even layer over the crumble. With your index finger, tuck the second strip of acetate between the cake ring and the top ¼ inch of the first strip of acetate, so that you have a clear ring of acetate 5" to 6" tall – high enough to support the height of the finished cake.
Place the two half circle cake pieces on top of the frosting layer and press in additional cake scraps to fill in the layer. Use the back of your hand to press the scraps together into a flat even layer.
Repeat steps 6, 7, and 8.
Place the remaining cake round onto the frosting and cover the top with remaining ⅕ frosting.
Top with remaining crumble. Place sheet pan into the freezer and freeze for a minimum of 12 hours. Cake will keep in the freezer for up to 2 weeks.
At least 3 hours before serving, remove the cake from the freezer and using your fingers and thumbs, pop the cake out of the cake ring. Gently peel off the acetate and transfer cake to platter or stand.
Let it defrost in the fridge or counter for a minimum of 3 hours (wrapped well in plastic, the cake can be refrigerated for up to 5 days).