Copycat Costco Vanilla Cake

For the Cake:

• 2 1/2 plus 2 tablespoons cake flour
• 1 1/2 cups plus 2 tablespoons granulated sugar
• 1 tablespoon baking powder
• 3/4 teaspoon fine salt
• 3/4 cup unsalted butter, very soft
• 4 large egg whites, room temperature
• 1 large egg yolk, room temperature
• 1 tablespoon vanilla extract
• 1 cup buttermilk, room temperature

For the Buttercream:

• 2 tablespoons water
• 2 tablespoons cornstarch
• 1 cup unsalted butter, very soft
• 1/2 cup vegetable shortening, room temperature
• 4 cups powdered sugar
• 1/2 teaspoon fine salt
• 1 tablespoon vanilla extract
• 3 to 4 tablespoons whole milk or heavy cream, room temperature

For Assembling the Cake:

• Copycat Costco Vanilla Mousse
• Sprinkles

To make the Cake: