Preheat the oven to 350°F, spray two 8-inch cake pans with baking spray, line the bottoms with parchment paper, and set them aside.

In a large mixing bowl, combine the cake flour, granulated sugar, baking powder, and salt, then stir briefly so the baking powder is evenly distributed.

With the mixer running on low, gradually add the very soft butter and mix until all of the butter is worked in and the mixture looks sandy; continue beating for 1 to 2 minutes, then scrape down the sides and bottom of the bowl.

Add the egg whites gradually, one at a time, beating well after each addition, then beat in the egg yolk and vanilla extract.

With the mixer on low, add the buttermilk in 3 additions, beating for 1 to 2 minutes after each addition, then divide the batter evenly between the prepared pans.

Bake for 26 to 30 minutes, until the tops are golden and a toothpick comes out mostly clean with a few moist crumbs; start checking at 26 minutes.

Let the cakes cool in their pans for 5 minutes, then invert them onto wire racks, remove the parchment, and let them cool completely; for advance assembly, wrap the cooled layers in plastic wrap and refrigerate them.

For the buttercream, whisk together the water and cornstarch in a small bowl and set it aside, then beat the softened butter and shortening in a large bowl for 3 to 4 minutes until light and fluffy, scraping the bowl as needed.

Mix in the cornstarch slurry, gradually add the powdered sugar with the mixer on low, scraping as needed, then add the salt and vanilla; if the frosting is too thick, add the milk or cream 1 tablespoon at a time, up to 4 tablespoons total, until spreadable but not droopy.

If the cake layers have domes, level them with a serrated knife, then put a small dab of buttercream on a cake board or serving plate and set the first layer on top.

Spoon some buttercream into a pastry bag, pipe a ring around the outer edge of the first layer to form a dam, and fill the center with the prepared vanilla mousse.

Set the second cake layer on top, frost the outside with the remaining buttercream using an offset spatula to smooth and remove excess frosting, then decorate with any remaining frosting, add sprinkles, and slice.