Preheat the oven to 350°F. Butter the bottom and side of the pan(s). Dust with coca powder to coat, then invert and tap out any excess. (If making cupcakes, use liners instead of greasing and coating the tins.)
Unsalted butter, ½ cup sifted unsweetened cocoa powder
Whisk together the ½ cup cocoa powder, cake mix, and baking soda in a large bowl. Stir in the yogurt, butter, espresso, and eggs until moistened and no lumps remain (be careful not to overmix). Divide the batter between the prepared pans.
2½ cups all-purpose flour, 1½ cups sugar, ¾ teaspoon baking soda, ¾ teaspoon baking powder, 1 teaspoon table salt, ½ cup sifted unsweetened cocoa powder, 4 cups dry Cake Magic! Cake Mix, ¾ cup full-fat plain yogurt, 1 cup unsalted butter, melted and cooled, or 1 cup vegetable oil, 1 cup brewed espresso, 4 large eggs, ¼ teaspoon baking soda
Bake until the layers are domed and a few moist crumbs cling to a skewer inserted in the center of the cake, 32 to 36 minutes (40 to 45 minutes for a Bundt, 25 to 35 minutes for a 13 by 9-inch cake, and 20 to 25 minutes for cupcakes). (At this point, coat the layers with syrup as directed and let them cool completely in the pans.)
While the cake is baking, prepare the syrup: Combine the sugar, stout beer, nutmeg and salt in a small saucepan and bring to a boil over medium-high heat. Stir to dissolve the sugar. Remove from the heat and set aside to cool. Use the syrup warm or let it stand, covered, until it reaches room temperature.
½ cup sugar, ½ stout beer, Pinch of salt, Pinch of ground nutmeg
After removing the cake layers from the oven, pierce them (still in the pans) at 1-inch intervals with a skewer or paring knife. Pour or brush the syrup over the layers, dividing it evenly. Set the layers aside, in the pans, on a wire rack to cool completely. (The syrup will soak into the cakes.)
When the cakes are cool and no longer wet to the touch, 1 to 2 hours, make the frosting: Combine the butter, malted milk powder, salt, and 2 cups of the sugar in a large bowl and beat with an electric mixer on low speed until incorporated, about 1 minute. Add the remaining sugar and beat on medium speed until the frosting is pale and no longer grainy, about 2 minutes. Add the vanilla. In a slow, steady stream, drizzle in the chocolate and beat until the frosting is very light and fluffy, about 2 minutes.
Pinch of salt, 1½ cups (3 sticks) unsalted butter, at room temperature, ¾ cup malted milk powder, 4 cups confectioner's sugar, 4 ounces semisweet chocolate, 1 teaspoon pure vanilla extract
Run a knife, or an offset spatula, around the edge of one of the cake layers to loosen it. (Turn the layers out of their pans 1 by 1, as you frost them.) Place the layer on a cake plate with strips of waxed, or parchment, paper underneath; these will catch any drips and keep your cake plate clean. Spread the layer with about a third of the frosting on top.
Frost the rest. Run a knife, or an offset spatula, around the edge of the remaining cake layer. Invert it onto the frosted layer. Frost the sides and top of the cake with the remaining frosting.
Notes
It’s important to use table salt in the cake mix; other types will eventually settle out of the mix.