Place oven rack on the middle setting and preheat the oven to 350ºF (177ºC). Grease and flour (or use homemade cake release) two 8" or 9" round cake pans.

In a large bowl with a handheld mixer or the bowl of a stand mixer fitted with the paddle attachment, combine the flour, cornstarch, sugar, baking powder, and salt. Blend ingredients together on low until completely combined, about 1 minute.

Add the room temperature butter and beat mixture on low speed until it resembles sand. Scrape down the sides and bottom of the bowl with a spatula and mix again on low for a few seconds until thoroughly combined.

Combine the whole milk, egg whites, vanilla extract, and cake batter extract in a large container with a spout and mix gently with a fork until blended.

With the mixer speed on medium-low, add half of the milk mixture (about 1 cup) to the crumb mixture and mix until blended. Add the remaining milk mixture to the batter and beat again on medium-low until everything is incorporated. Turn the mixer off, scrape down the sides and bottom of the bowl with a spatula, and increase mixer speed to medium. Beat the batter for about 30 seconds until batter is smooth.

Remove the bowl from the mixer and use a spatula to gently fold in the sprinkles until evenly dispersed.

Divide batter evenly between the two prepared cake pans. Bake cakes for 26-28 minutes or until a toothpick inserted in the center comes out clean and the tops are lightly browned. Remove from oven and allow cakes to cool in the pans completely on a wire rack before removing and assembling.

In a large bowl with a handheld mixer or a stand mixer fitted with the paddle attachment, beat the butter on medium-high until smooth, about 2 minutes.

Reduce the mixer to low, then slowly add the powdered sugar and mix until everything is combined. Add the vanilla extract and mix again.

Slowly add the milk or cream, watching for and stopping at desired consistency. Add the salt, then increase the mixer speed to medium and beat until completely smooth.

If after beating, the frosting is too stiff, add more cream. If it is too thin, add more powdered sugar, about 2 Tablespoons at a time. Turn off the mixer and use a spatula to stir the buttercream by hand to remove air bubbles and make it smooth and creamy.

Place one layer, bottom side up, on a plate or cake stand. Using a spatula or knife, spread an even layer of frosting over the entire surface. Place the second layer on top, bottom side down, and press down on the top lightly. Spread the rest of the frosting on the cake, beginning with the top and working your way down the sides. Decorate as desired.