12Tablespoons(171g) unsalted buttersoftened to room temperature1
1cup(240mL) heavy creamroom temperature1,2
6large egg whitesroom temperature1
2 and ½teaspoonsvanilla extract
½cup(113g) unsalted buttersoftened to room temperature
2cups(240g) powdered sugar
1 to 2Tablespoons(15-30mL) milk or cream
pinch of salt
Place oven rack on the middle setting and preheat the oven to 350ºF (177ºC). Grease and flour (or use homemade cake release) a 9" x 13" baking pan. Set aside.
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. 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 the 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.
Pour the batter into the prepared cake pan. Bake cake for about 32-34 minutes or until a toothpick inserted in the center comes out clean and the top is lightly browned. Remove from oven and allow cake to cool completely on a wire rack before frosting.
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 extract, and salt. Add the cream and continue to mix until everything starts to come together.
Increase mixer speed to medium and beat until completely combined, scraping down sides as needed. Add salt to taste. Spread evenly over cooled cake and decorate as desired. Store leftovers covered tightly at room temperature up to 4 days or in the refrigerator up to 1 week. Cake freezes well, up to 2 months. Thaw at room temperature.