Preheat oven to 300F and line a baking sheet with parchment or a silpat.
In the bowl of a stand mixer fitted with the paddle attachment, combine all dry ingredients (sugars, flour, baking powder, salt, sprinkles). Mix on low until well combined.
Add oil and vanilla, mix again on low to distribute until small clusters form.
Bake for 15mins.
Cool completely before using on cake.
Store in an airtight container for 1 week at room temperature or 1 month in the fridge or freezer.
Preheat oven to 350F and spray a quarter-sheet pan (10”x15”) cooking spray and line with parchment or a 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 fitted with the paddle attachment, combine butter, shortening, and sugars. Beat on med-high for 2-3mins. Scrape bowl if necessary.
Add eggs one at a time, fully incorporating after each addition. Scrape bowl.
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. Scrape bowl.
On low speed, add flour mixture and mix just until batter comes together, 30-60 secs.
Spread cake batter in an even layer in the pan and sprinkle with remaining 25g (2 Tbsp) rainbow sprinkles.
Bake for 30-35mins or until a cake tester comes out clean.
Cool completely on a wire rack.
The cooled cake can be stored in the fridge, wrapped in plastic wrap, for up to 5 days.
In a medium bowl, sift together confectioners sugar, salt, and baking powder. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, combine butter, shortening, and cream cheese. Beat on med-high for 2-3mins until the mixture is smooth and fluffy. Scrape bowl.
Turn mixer to low and stream in glucose, corn syrup, and vanilla. Increase mixer to med-high and beat for 2-3mins until smooth and glossy white. Scrape bowl.
Turn mixer to low and add in sugar mixture and citric acid (I used lemon juice). Once combined, turn mister to med-high and leaf for 2-3mins until white and smooth.
Use immediately, or store it in an airtight container in the fridge for up to 1 week.
Whisk together the milk and vanilla in a small bowl or measuring cup.
Cut acetate into two 3” high and 20” long strips.
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 (start in the corners to be able to get 2 full ones). These will be your top and bottom layers. Stamp out 2 half circles from the remaining scraps.
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 1/5 of the frosting in an even layer over the cake.
Sprinkle 1/3 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 1/5 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 1/4 inch of the first strip of acetate, so that you have a clear ring of acetate 5 to 6 inches 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 1/5 frosting. Spread frosting evenly or a bit messier if you like.
Top with remaining crumble — either around the edge or in the center, however you like.
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).