I like to bake this recipe in a Glass Baking Pan, but you can use a 13 x 9 Cake Pan if you prefer. This recipe incorporates my homemade substitution for cake flour. Due to food allergies, it is difficult to find cake flour at the grocery store that is safe for our family. A few years ago, I discovered that I could easily make my own cake flour substitute using All-purpose flour and cornstarch to deliver the benefits of cake flour’s lower protein content without adding allergens. If you would prefer to use premixed cake flour, simply use 2 full cups of cake flour instead of the All-purpose flour and cornstarch called for in the recipe. If you only have All-purpose flour on hand, you can use 2 full cups of All-purpose flour and your recipe will still taste delicious.
Preheat the oven to 350 degrees Fahrenheit and position an oven rack in the middle of your oven. Apply a thin coating of Baking Pan Release Spread, pan spray, or butter to your baking pan.
In a small saucepan, melt the butter over low to medium heat. When the butter has melted, add the cocoa powder and espresso powder to the pan. Stir until it becomes a smooth mixture. Remove from the heat and allow to cool.
In a medium bowl, combine the flour, cornstarch, baking soda, and salt. Use a dry whisk to mix the dry ingredients and break up any small lumps. You can also sift the dry ingredients to achieve the same result.
Add the eggs and sugar to a large mixing bowl. Using a whisk or a mixer, beat the sugar and eggs until they are thick, smooth, and pale yellow in color. This takes about two minutes and is often called the “ribbon” stage. When you lift the whisk or beater from the bowl, the mixture clinging to it will fall to the bowl below in a smooth ribbon.
Add the warm butter mixture to the bowl in a steady stream while mixing to temper the mixture and prevent the eggs from becoming grainy.
Add the dry ingredients in one addition to the mixing bowl. Mix slowly just until the dry ingredients have completely integrated into the batter. This won't take long. Take great care not to overmix the dough. Mixing develops the gluten in the flour and overmixing will encourage the cake's texture to become tough.
Add the vanilla and buttermilk to the mixing bowl and whisk until it is completely incorporated and the batter is smooth. Again, take care not to overmix.
Transfer the dough to your prepared pan, spreading the dough to evenly cover the bottom surface of the pan evenly. This cake batter is very thin by design to create a light cake. Don’t be concerned that the batter is thin, it will bake up beautifully.
Transfer the pan to the warm oven and bake for 30 minutes. When the cake is fully baked, the edges will begin to pull away from the sides of the pan and the top of the cake will take on a matte appearance. A toothpick inserted into the middle of the pan should come out cleanly or with small crumbs attached when the cake is finished baking.
Remove the pan from the oven. Allow the cake to cool to room temperature before frosting.
For the Frosting:
Place the softened butter and shortening in a large mixing bowl. Beat until the mixture is completely smooth. Add the vanilla and cocoa and beat to combine. Add the powdered sugar in 1/2 cup increments, beating slowly after each addition to combine. Turn the mixer up to high and beat for 1-2 minutes. This will help create a smooth and airy buttercream. Add milk or heavy cream as needed to help achieve a smooth, spreadable consistency.
Spread the buttercream on the surface of the cooled cake or cupcakes. If necessary, this buttercream can be thinned with the addition of more heavy cream to make it easier to spread without scarring the surface of the cake. Any leftover frosting can be stored in an airtight container for several days.
Notes
This cake stores very well covered at room temperature. It’s so delicious that it never lasts very long here at the farmhouse, but it can be kept at room temperature for several days. The last piece will taste just as delicious as the first.