Step 1: Bake the Crumble
Let’s get crumbly—crank your oven to 300°F and line a baking sheet with parchment (I love that prep buzz!). In a stand mixer with the paddle, mix 100 g granulated sugar, 25 g packed light brown sugar, 90 g cake flour, 2 g baking powder, 2 g kosher salt, and 20 g rainbow sprinkles on low—blend it well (my kitchen’s festive already!). Add 40 g grapeseed oil and 12 g clear vanilla extract—mix on low ‘til clusters form (I grin at those clumps!). Spread on the sheet—bake 15 minutes (my heart races with that sprinkle scent!). Cool completely—store airtight (I sneak a nibble—it’s party time!).

Step 2: Preheat and Prep for Cake
Bump the oven to 350°F—spray a 10”x15” quarter-sheet pan with cooking spray and line with parchment (I’m ready for Funfetti!). Take a sec to breathe—this is gonna be a sprinkle wonder, and you’re on your way.

Step 3: Mix the Dry Cake Goods
In a medium bowl, whisk 245 g cake flour, 6 g baking powder, 3 g kosher salt, and 50 g rainbow sprinkles—stir well (I love that sprinkle pop!). Set aside—this is the fluffy base!

Step 4: Cream the Cake Base
In the stand mixer with the paddle, beat 55 g room-temp unsalted butter, 60 g vegetable shortening, 250 g granulated sugar, and 50 g packed light brown sugar on medium-high for 2-3 minutes—scrape the bowl (I love that fluff!). Add 3 large eggs one at a time—beat well after each, scrape again (my kitchen smells like a party!). Stream in 110 g buttermilk, 65 g grapeseed oil, and 8 g clear vanilla extract on low—crank to medium-high and beat 4-6 minutes ‘til light and streak-free (I’m drooling already!).

Step 5: Finish the Batter
On low, add the dry mix—beat 30-60 seconds ‘til just together (I keep it gentle!). Spread in the pan—sprinkle 25 g rainbow sprinkles on top (my heart’s racing with color!). Bake 30-35 minutes—check with a tester ‘til clean (I watch with glee as it puffs!). Cool on a rack—wrap and chill if needed (that sprinkle scent’s pure magic!).

Step 6: Whip the Frosting
Sift 300 g powdered sugar, 3 g kosher salt, and 0.38 g baking powder into a bowl—set aside (I love that fluffy prep!). In the stand mixer with the paddle, beat 172.5 g room-temp unsalted butter, 75 g vegetable shortening, and 82.5 g cream cheese on medium-high 2-3 minutes—scrape the bowl (I grin at that silk!). Stream in 37.5 g glucose, 27 g corn syrup, and 18 g clear vanilla extract on low—crank to medium-high and beat 2-3 minutes ‘til glossy (my kitchen’s frosting heaven!). Add sugar mix and 0.38 g citric acid (or ¼ tsp lemon juice)—beat on low, then medium-high 2-3 minutes ‘til white and smooth (I sneak a taste—it’s bliss!). Use now or chill—ready to frost!

Step 7: Prep the Soak
Whisk 55 g milk and 4 g clear vanilla extract in a small bowl—set aside (I love that sweet soak!).

Step 8: Assemble the Cake
Cut two 3”x20” acetate strips—line a 6-inch cake ring on a parchment-lined quarter-sheet pan (I’m giddy for layers!). Invert the cooled cake on parchment—peel off the lining (my heart races with each step!). Stamp out 2 full 6-inch circles and 2 half-circles from scraps—clean the ring (I feel pro cutting!). Place one full layer in the ring—brush with soak (I love that damp shine!). Spread ⅕ frosting with a spoon—sprinkle ⅓ crumble and press (I save chunky bits for later!). Add another ⅕ frosting—tuck the second acetate strip to extend the height (my tower grows!). Layer the half-circles and scraps—press flat, soak, spread ⅕ frosting, add ⅓ crumble (I’m stacking joy!). Top with the last full layer—soak, spread ⅕ frosting, add final crumble (that first bite’s coming!).

Step 9: Freeze and Finish
Freeze 12 hours—pop out the ring and peel acetate after (I love that reveal!). Defrost in the fridge 3 hours—slice and serve (pure bliss, and you made it happen!). Store wrapped—I sneak bites ‘til it’s gone!