12Tablespoons(1 and 1/2 sticks) unsalted buttersoftened to room temperature1
3/4cupheavy creamroom temperature1,2
6large egg whitesroom temperature1
1cup(2 sticks) unsalted buttersoftened to room temperature
1Tablespoonmilk or heavy cream
In a small saucepan over medium-high heat, bring the champagne to a boil.
Once boiling, reduce to medium heat and allow to simmer until reduced down to 1/2 cup, about 6 to 8 minutes. Set aside to cool completely to room temperature.
Place oven rack on the middle setting and preheat the oven to 350ºF. 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 (see photo in post text). 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 1/4 cup of the champagne reduction, heavy cream, egg whites, and vanilla extract in a large container with a spout (like a 2-cup glass measuring cup) 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.
Divide batter evenly between the two prepared cake pans. Bake cakes for about 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 completely on a wire rack before assembling.
In a large bowl with a handheld mixer, or a stand mixer fitted with the paddle attachment, beat the butter on medium speed until creamy, about 2 minutes.
With the mixer on low, add in the powdered sugar, vanilla, and remaining 1/4 cup of the champagne reduction. Increase mixer speed to medium, and beat until completely smooth, about 3 minutes. Add the milk or cream and beat again. If frosting is too stiff, add more milk or cream. If it is too thin, add more powdered sugar.
ASSEMBLE THE CAKE
Trim cooled cake layers to create a flat surface. You can do this with a large serrated knife or a cake leveler. Place one layer on a plate or cake stand and cover the top with buttercream. Spread evenly with an offset spatula.
Place second layer on top, cut surface down. For a sturdier cake, refrigerate at this point for about 10 minutes. When cake is slightly chilled, remove from refrigerator and spread more frosting on the top and around the sides. Serve. Leftovers stay fresh, covered tightly, at room temperature up to 5 days and in the refrigerator up to 1 week.
Make ahead: prepare cakes and frosting up to 1 day in advance. Cover cakes tightly and keep at room temperature or in the refrigerator until ready to assemble. Refrigerate prepared frosting in an airtight container until ready to use. Unfrosted cake layers may be frozen, wrapped tightly, up to 2 months. Thaw overnight in the refrigerator before assembling. Frosted cake can be frozen, wrapped tightly, up to 2 months. Thaw overnight in the refrigerator and bring to room temperature before serving.
It is imperative to use room temperature ingredients in this recipe. Allow ingredients to sit out at room temperature at least 45 minutes before beginning.
I strongly encourage using cream in this recipe. It keeps the crumb moist and dense and adds a creaminess to the overall texture of the cake.