Position oven rack in the center and heat to 350°F (175°C), then line a 9×13 inch (23×33cm) light-colored aluminum baking pan with parchment paper leaving overhang on both short ends, and grease the exposed long-side walls.

In a large bowl, whisk together 2¾ cups (325g) cake flour, 1 tablespoon (12g) baking powder, and ½ teaspoon (3g) fine sea salt for 30 seconds until evenly combined and set aside.

In a stand mixer fitted with the paddle attachment, beat 1 cup (225g) softened unsalted butter on medium speed for 2 minutes, then add 1½ cups (300g) granulated sugar and beat on medium-high for a full 5 minutes until pale ivory, nearly white, and fluffy.

With the mixer on medium-low, add 5 large room-temperature egg whites one at a time waiting 20 seconds between each addition, then add 2 teaspoons (10ml) vanilla extract and 1 teaspoon (5ml) almond extract and beat for 30 more seconds until the batter is smooth and glossy.

With the mixer on low, alternate adding the flour mixture and 1 cup (240ml) whole milk in five additions starting and ending with flour, mixing until just incorporated after each addition and stopping when a few faint flour streaks remain.

Remove the bowl from the mixer and fold in ½ cup (75g) rainbow sprinkle mix by hand using a silicone spatula with 10 to 12 wide strokes, then immediately transfer the batter to the prepared pan and spread into an even layer reaching all four corners.

Bake on the center rack for 28–32 minutes until the edges are pale golden and just pulling from the sides of the pan and the center springs back immediately when pressed lightly — do not open the oven before 25 minutes.

Cool in the pan on a wire rack for 30 minutes, then lift out using the parchment overhang and cool on the rack for a further 60 minutes until completely room temperature before frosting.

In the clean stand mixer bowl with the paddle attachment, beat 1½ cups (340g) softened unsalted butter on medium speed for 3 full minutes until pale and fluffy, then add 5 cups (600g) sifted powdered sugar in three additions on low speed followed by 1 tablespoon (15ml) vanilla extract and a pinch of salt, then add 3–4 tablespoons (45–60ml) heavy cream and beat on medium-high for 2 minutes until light and smooth.

Spoon buttercream onto the cooled cake and spread with an offset spatula in large circular sweeps to create swoop texture approximately ½ inch thick, then immediately scatter rainbow sprinkles across the frosted surface in two stages 10 minutes apart for layered coverage, and slice into a 4×3 grid for 12 squares.