1. Preheat your oven to 350°F (175°C). Grease your cake pans and line them with parchment paper for easy removal.

2. In a large bowl, whisk together the cake flour, baking powder, and salt until well combined.

3. In a separate bowl, use a hand mixer or stand mixer to beat the softened butter and sugar until light and fluffy, about 2-3 minutes.

4. Beat in the egg whites and egg yolk, one at a time, until fully incorporated. Then, stir in the vanilla extract.

5. Gradually add the dry ingredient mixture to the butter mixture, alternating with the buttermilk, mixing until just combined and smooth.

6. Pour the batter into your prepared cake pans and bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean. Allow the cakes to cool completely before frosting.

7. In a small saucepan, whisk together the water and cornstarch over medium heat until thickened. Set aside to cool slightly.

8. Using a stand mixer or hand mixer, beat the softened butter and shortening together until creamy and combined.

9. Gradually add the powdered sugar, salt, and vanilla extract, alternating with the cooled cornstarch mixture and milk or cream, until the frosting is light and fluffy.

10. Spread a generous layer of buttercream on the first cake layer, then top with a layer of the Costco-inspired vanilla mousse. Place the second cake layer on top.

11. Use the remaining buttercream to frost the entire cake, creating a smooth, even finish.

12. Finish your homemade Costco Vanilla Cake by adding sprinkles or other decorations as desired.