For the Chocolate Sour Cream Cake: (makes one 9-inch, 4-layer cake) - 3/4 cup sour cream, at room temperature - 1/2 cup plus 3 tablespoons cocoa powder (equal parts unsweetened cocoa powder and Dutch-processed)- - 2 large eggs, at room temperature - 2 large egg yolks, at room temperature - 1 1/2 teaspoons pure vanilla extract - 2 1/4 cups cake flour - 1 teaspoon baking powder - 1 teaspoon baking soda - 3/4 teaspoon kosher salt - 1 3/4 cups granulated sugar - 1 cup (2 sticks) unsalted butter, at room temperature - 1/2 cup water, boiling For the Chocolate Buttercream Frosting: (makes 6 cups, or enough for one 9-inch cake) - 6 ounces cream cheese, at room temperature - 4 teaspoons pure vanilla extract - 1 cup (2 sticks) unsalted butter, at room temperature - 1 cup unsweetened cocoa powder - 1 tablespoon plus 1 teaspoon light corn syrup - 7 cups confectioner's sugar - 3 to 6 tablespoons whole milk, at room temperature --- Recipe For the Chocolate Sour Cream Cake: 1) Preheat the oven to 350 (F). Coat two 9-inch round pans with nonstick cooking spray, and line the bottoms of the pans with parchment paper. 2) In a medium bowl, mix together 3/4 cup sour cream, 1/2 cup plus 3 tablespoons cocoa powder, 2 eggs, 2 egg yolks, and 1 1/2 teaspoons vanilla until smooth. 3) In the bowl of a stand mixer fitted with a paddle attachment, beat together 2 1/4 cups cake flour, 1 teaspoon baking powder, 1 teaspoon baking soda, 3/4 teaspoon salt, and 1 3/4 cups granulated sugar on low speed just until blended. 4) Add 1 cup unsalted butter and half of the cocoa mixture (from the second step) to the flour mixture (from the third step) and beat until the dry ingredients are moist. 5) Increase the mixer speed to medium and beat until smooth, about 2 minutes. Gradually add the remaining cocoa mixture in 2 batches, beating well after each addition. Add 1/2 cup boiling water and beat until smooth. 6) Divide the batter equally between the prepared pans and smooth the tops with an offset spatula. Bake until a wooden pick inserted into the center comes out clean, about 30 to 35 minutes. Let the cakes cool in the pans for 10 minutes before turning them out onto a wire rack to cool completely. Remove the parchment paper. For the Chocolate Buttercream: 1) In the bowl of a stand mixer fitted with the paddle attachment, beat 6 ounces cream cheese and 4 teaspoons vanilla together on low speed until smooth. Increase the mixer speed to high. Add 1 cup butter and beat until incorporated, using a rubber spatula to scrape the bowl as necessary, about 1 minute. 2) Reduce the mixer speed to low and beat in 1 cup cocoa powder and 1 tablespoons plus 1 teaspoon light corn syrup. Add 7 cups confectioners' sugar and beat until blended. Add 3 to 6 tablespoons of whole milk, one tablespoon at a time, beating until the buttercream achieves a spreadable consistency. Set aside until ready to use.