Step 1

Place 1 1/2 sticks of the unsalted butter, 1/3 cup buttermilk, 3 large eggs, and 8 ounces cream cheese on the counter. Let everything sit at room temperature until the butter and cream cheese are softened, about 1 1/2 hours.

Step 2

About 30 minutes before the butter and cream cheese are ready, arrange a rack in the middle of the oven and heat the oven to 350°F. Coat a 8x11-inch glass or ceramic baking dish with cooking spray and line the bottom and two long sides with a sheet of parchment paper to form a sling.

Step 3

Prepare the bottom layer: Place 2 cups of the all-purpose flour, 1 cup of the granulated sugar, 1/4 cup cornstarch, 2 teaspoons baking powder, and 1 teaspoon of the kosher salt in the bowl of a stand mixer (or large bowl if using an electric hand mixer). Whisk to combine.

Step 4

Melt the remaining 1 stick unsalted butter in the microwave or on the stovetop. Add the melted butter, buttermilk, and 1/2 cup vegetable oil to the flour mixture. Beat with the whisk attachment on low speed until combined, scraping down the sides of the bowl with a flexible spatula as needed, about 2 minutes total.

Step 5

Beat in 2 of the eggs on medium speed one at a time, beating well after each addition. Add 1 teaspoon of the vanilla extract and 1/2 teaspoon almond extract, and beat on medium speed until combined. Add 1/2 cup of the rainbow sprinkles and fold in by hand with the spatula. Transfer the batter to the baking dish and spread into an even layer.

Step 6

Prepare the top layer: Wash and dry the mixer bowl. Place the 1 1/2 sticks room temperature unsalted butter, cream cheese, remaining 1 cup granulated sugar, 1/2 cup powdered sugar in the bowl, and the remaining 1/2 teaspoon kosher salt in the bowl.

Step 7

Beat with the whisk attachment on medium speed until lightened in color and fluffy, scraping down the sides of the bowl with a flexible spatula as needed, 4 to 5 minutes total. Add the remaining egg and 1 1/2 teaspoons vanilla extract and beat until combined.

Step 8

Turn off the mixer. Scrape down the sides and bottom of the bowl. Fold in the remaining 3/4 cup all-purpose flour with a flexible spatula, then fold in the remaining 1/2 cup rainbow sprinkles until well distributed. Dollop over the bottom layer and use an offset spatula to spread into an even layer.

Step 9

Bake until the edges are set and the cake is golden brown but still quite loose in the center, 35 to 40 minutes (it will appear almost liquid under the surface in spots, but will quickly set upon cooling). Place on a wire rack and let cool for 15 minutes.

Step 10

Run a thin knife around the edges of the cake to loosen. Grasping the parchment, lift the cake out of the baking dish and place on the wire rack. Let cool completely, about 2 1/2 hours. Place some powdered sugar in a fine-mesh strainer and dust it over the top of the cake before slicing and serving.