As with the high ratio yellow cake recipe, emulsification is king here. The recipe is built accordingly, making a process allowance for the cocoa powder. An interesting feature of cocoa powder is that it delivers double the chocolate flavor if you combine it with boiling milk or water before you add it to your mix (another trick from the world of box cake formulators). Since I can’t abide wasting perfectly good chocolate flavor, I’ll be doing that.
I should add that this cake can be made with butter if you wish. As it is it won’t be exactly like a commercial cake because most of us don’t have access to emulsified shortening or high ratio flour, which are usually used to make high ratio cakes. But butter will work here. For more moisture you can substitute oil for some of the fat (olive oil is best since it also adds emulsifiers and tightens the crumb. This recipe will make three 8″ round layers or one 11″ x 14″ x 2″ sheet cake (I should add that if you’re baking in a pan of that size, but the soda back by 1/4 teaspoon to help ensure the cake doesn’t fall in the middle)
12 ounces (1 1/2 cups) boiling water
1.65 ounces (1/2 cup) cocoa powder
1 tablespoon vanilla extract
15 ounces (3 1/3 cups + ) cake flour
1 1/2 teaspoons salt
1 tablespoon baking powder
1 1/2 teaspoons baking soda (1/4 tsp. less if you’re making a large sheet cake)
15 ounces (2 cups, 2 tablespoons) sugar
6.75 ounces (1 cup) shortening
6 egg yolks
Preheat your oven to 350 degrees. Pour the water into a small saucepan and bring to a simmer. Whisk in the cocoa powder until it’s completely incorporated and allow the mixture to cool. Add the vanilla. Sift the flour into a large bowl, add the salt, baking powder, baking soda and HALF the sugar. Set that aside also. Next, combine the remaining sugar and shortening in the bowl of a stand mixer fitted with the paddle (beater) and cream them thoroughly, about two minutes. Beat in the eggs one at a time, then the yolks all at once. Stir in about a third of the flour, then half the chocolate mixture, another third of the flour, etc. until everything is incorporate. Scrape the mixture into a greased, parchment-lined 9″ x 13″ cake pan and bake on a middle rack for about half an hour until the cake springs back in the middle when touched.