Rainbow Sprinkle Birthday Cake

For the cake

16 tablespoons (226 grams; 2 sticks) unsalted butter, at room temperature, plus more for greasing the pans

2½ cups (357 grams) flour

¼ cup (35 grams) cornstarch

1 teaspoon (4 grams) kosher salt

2 teaspoons (9 grams) baking powder

1½ cups (324 grams) granulated sugar

4 large egg whites, at room temperature

¼ cup neutral oil, such as vegetable or canola

1 tablespoon pure vanilla extract (can substitute imitation vanilla extract)

½ teaspoon almond extract

¾ cup whole milk, at room temperature

½ cup rainbow sprinkles, plus more for decorating (see notes)

For the frosting

1 pound white chocolate, coarsely chopped

1 pound plus 3 ounces cream cheese, left at room temperature for 30 minutes

12 tablespoons (170 grams; 1½ sticks) unsalted butter, cut into 6 pieces and left at room temperature for 30 minutes

3 tablespoons (about 1 ounce) sour cream or crème fraîche

¾ teaspoon pure vanilla extract

For the cake: Preheat the oven to 350 degrees. Grease three 8-inch cake pans with butter and line the bottoms with parchment paper. (You can bake in batches if you have fewer pans.)

Whisk together the flour, cornstarch, salt and baking powder in a medium bowl.

Combine the butter and sugar in the bowl of a stand mixer fitted with the paddle attachment. Beat for 3 to 4 minutes on medium speed until light and fluffy. Add the egg whites, one at a time, mixing well after each addition. Add the oil and vanilla and almond extracts. Stop to scrape down the bowl.

On low speed, alternately add the flour mixture and the milk in two or three additions, mixing until just barely incorporated.

Use a rubber spatula to gently fold in the sprinkles until evenly distributed. Divide the batter evenly among the cake pans, smoothing the surface. (The total weight of the batter is about 1,330 grams, or about 443 grams per cake pan.)

Bake (middle rack) for 20 to 25 minutes, until a toothpick inserted into the center comes out mostly clean (with maybe a moist crumb or two). The cake will be lightly golden brown in spots, and the edges should have pulled away from the sides of the pan. Let cool in the pans for 10 minutes and then turn onto a wire rack to cool completely.

For the frosting: Place the chocolate in a heatproof bowl and melt in the microwave on HIGH in 15-second bursts, stirring in between, until the chocolate is more than three-quarters melted. Stir without further heating until the chocolate is completely fluid. Let cool until just slightly warm to the touch (80 to 85 degrees).

Combine the cream cheese, butter and sour cream or crème fraîche in the bowl of a stand mixer fitted with the paddle attachment. Beat on medium speed for 3 to 4 minutes, until smooth and creamy.

Stop to scrape down the sides of the bowl. Add the melted/cooled white chocolate; beat on low speed to incorporate. Add the vanilla extract; beat on low to form a well-blended frosting. It should be fairly thick, yet spreadable.

To assemble the cake, place a dab of frosting in the middle of a 9- or 10-inch cardboard cake round or plate. Place one cake layer in the center, flatter side up. (You could also just place the dab of frosting and cake directly on a large plate, ideally with little or no rim.)

Use an offset spatula or spoon to spread a generous ½ cup frosting over the top of the bottom cake layer, making sure it reaches the edges. Place the next cake layer on top, also flatter side up. Repeat with another layer of frosting. Lay the final cake layer on top. Place a small amount of frosting (at least ½ cup) in a separate bowl for the crumb coat, which is your base layer of frosting that will help seal in the crumbs and give you a smooth surface to which you can apply the rest of the frosting. Use an offset spatula or table knife to apply the thin crumb coat all over the top and sides of the cake. Refrigerate for 20 to 30 minutes, to let the crumb coat set.

Transfer the cake to a cake stand or serving plate, sliding it off the cardboard cake round, if desired. Apply the remaining frosting to the cake, and decorate with more sprinkles. If the frosting on the cake gets too soft, return the cake to the refrigerator to let it firm back up for another 20 or 30 minutes.

Notes: If you don't have a stand mixer, a hand mixer and large bowl can get the job done.

The baked cake layers can be made up to a day in advance and stored, tightly wrapped, at room temperature. The frosting keeps up to a week in the refrigerator; freeze for long-term storage. Let the frosting come to room temperature before using (at least a few hours). You may need to smooth it out with the mixer or a wooden spoon. The frosted cake can be wrapped and refrigerated overnight.

Serves 8 to 10

Nutrition Calories: 670; Total Fat: 46 g; Saturated Fat: 25 g; Cholesterol: 100 mg; Sodium: 250 mg; Carbohydrates: 63 g; Dietary Fiber: 0 g; Sugars: 43 g; Protein: 7 g.

Cake adapted from a recipe by Molly Yeh at mynameisyeh.com; frosting adapted from a recipe by Rose Levy Beranbaum at kingarthurflour.com.