Ingredients:
Cake:
4 large eggs plus 2 large yolks
1/2 cup buttermilk
1 tablespoon vanilla extract
2 1/4 cups bleached cake flour
1 3/4 cups granulated sugar
1 1/4 teaspoons baking powder
1/4 teaspoon baking soda
1/2 teaspoon table salt
8 tablespoons unsalted butter, softened
1/2 cup vegetable oil
Frosting:
2 1/4 cups confectioners’ sugar
1/2 cup unsweetened coco powder
8 tablespoons unsalted butter, softened
1/4 cup hot water
1/4 teaspoon table salt
2 ounces bittersweet chocolate, melted
Directions:
Cake:
Adjust oven rack to middle position and heat oven to 350°. Grease and flour 13×9-inch baking pan. Combine eggs and yolks, buttermilk, and vanilla in 2-cup liquid measuring cup and whisk with fork until smooth.
Combine flour, sugar, baking powder, baking soda, and salt in bowl of stand mixer fitted with paddle. Mix on low speed until combined about 20 seconds. Add butter and oil ad mix on low speed until combined, about 30 seconds. Increase speed to medium and beat until lightened, about 1 minute. Reduce speed to low and, with mixer running, slowly add egg mixture. When mixture is fully incorporated, stop mixer and scrape down bowl and paddle thoroughly. Beat on medium-high speed until batter is pale, smooth, and thick, about 3 minutes. Transfer batter to prepared pan and smooth top. Tap pan firmly on counter 5 times to release any large air bubbles.
Bake until toothpick inserted in center comes out with few crumbs attached, 28 to 32 minutes. Let cake cool completely in pan on wire rack, about 2 hours.
Frosting:
Combine sugar, cocoa, butter, hot water, and salt in bowl of stand mixer fitted with whisk attachment. Mix on low speed until combined, about 20 seconds. Increase speed to medium and continue to mix smooth, about 1 minute longer, scraping down bowl as necessary. Add melted chocolate and whip on low speed until incorporated. Let sit at room temperature until thickened to spreadable consistency, 30 to 40 minutes.
Frost cake. Refrigerate until frosting is set, about 20 minutes, before serving.
