Step 1

For the cake: Arrange a rack in the center of the oven and preheat to 350 degrees F. Lightly coat two 9-inch cake pans with nonstick cooking spray. Line the bottoms with parchment paper rounds; lightly coat the paper with nonstick spray.

Step 2

Whisk together the flour, cornstarch, baking powder, salt and baking soda in a large bowl until combined. Beat the butter, granulated sugar, oil and vanilla with an electric mixer on medium-high speed until light and fluffy, 3 to 4 minutes. Add the eggs and yolks one at a time, beating to blend between additions and occasionally scraping down the sides and bottom of the bowl until the batter is completely combined, smooth and creamy. With the mixer on low, add the dry ingredients in 3 additions, alternating with the buttermilk, beginning and ending with the dry ingredients.

Step 3

Scrape the batter evenly into the prepared pans. Smooth the tops. Bake, rotating the pans from left to right halfway through, until the cakes are a light golden brown and a tester inserted into the centers comes out clean, 35 to 45 minutes. Transfer the pans to a wire rack and let the cakes cool for 30 minutes. Run a knife around the sides of the cakes and invert onto the wire rack. Remove the parchment and let the cakes cool completely.

Step 4

For the frosting: Beat the butter in a medium bowl with an electric mixer on high speed until smooth, about 1 minute. Reduce the speed to low and gradually mix in the confectioners' sugar until completely smooth. With the motor running, drizzle in the heavy cream, vanilla and salt. Increase the speed to high and beat until the frosting is light and fluffy, about 2 minutes.

Step 5

To assemble the cake: Place one cake, domed-side down, on a platter. Spread 1 1/2 cups of the frosting evenly over the top. Place the remaining cake, domed-side up, on top. Spread the top with 1 1/2 cups of the frosting. Frost the sides of the cake with 1 1/2 cups of the frosting and smooth out completely to make an even crumb coat. (It's ok if some of the cake shows through. This is just a base layer.) Refrigerate for 30 minutes to let the frosting set.

Step 6

Spread the remaining frosting over the top and sides, smoothing it out.