Golden Buttermilk Cake with Silky Chocolate Icing

1. Preheat Oven and Prepare Pan

Preheat your oven to 350°F and adjust the rack to the middle position. Grease a 13 by 9-inch baking pan and dust it with flour to prevent the cake from sticking.

2. Mix Egg Mixture

In a 2-cup liquid measuring cup, whisk together the large eggs, large egg yolks, buttermilk, and vanilla extract until smooth. Set aside.

3. Combine Dry Ingredients

In the bowl of a stand mixer fitted with the paddle attachment, combine the bleached cake flour, granulated sugar, baking powder, baking soda, and table salt. Mix on low speed for about 20 seconds to ensure everything is evenly distributed.

4. Add Butter and Oil

Add the softened unsalted butter and vegetable oil to the dry ingredients. Beat on low speed for 30 seconds until the mixture starts to come together. Increase the speed to medium and continue beating for about 1 minute until the batter lightens in color and becomes fluffy.

5. Incorporate Egg Mixture

With the mixer running on low speed, slowly pour in the egg mixture. Once all the liquid is added, increase the speed to medium-high and beat for another 3 minutes to incorporate air into the batter.

6. Bake the Cake

Transfer the batter into the prepared pan, smoothing the top with a spatula. Tap the pan on the counter a few times to release air bubbles. Bake for 28 to 32 minutes, or until a toothpick inserted in the center comes out with just a few crumbs attached. Let the cake cool completely on a wire rack, about 2 hours.

7. Prepare Frosting

In the bowl of a stand mixer fitted with the whisk attachment, combine the confectioners' sugar, unsweetened cocoa powder, softened unsalted butter, hot water, and table salt. Mix on low speed for 20 seconds to combine. Increase the speed to medium and beat for about 1 minute until smooth, scraping down the sides of the bowl as necessary. Add the melted bittersweet chocolate and whip on low speed until fully incorporated.

8. Thicken Frosting

Let the frosting sit at room temperature for 30 to 40 minutes to thicken slightly, making it easier to spread.

9. Frost the Cake

Once the cake is completely cool, frost it with the chocolate icing. Spread the frosting evenly over the top and sides of the cake. To set the frosting and make slicing easier, refrigerate the cake for about 20 minutes until the frosting feels firm to the touch.