Read all steps and notes before beginning.
Preheat oven to 400°F. Spray a 12x17-inch XL baking sheet with nonstick baking spray. Set aside.
In a medium-sized saucepan, melt the butter, coffee, and cocoa powder on the stove over medium heat. Bring it to a boil, then pour the mixture into a large bowl. Set aside.
1 cup unsalted butter, 1 cup brewed black coffee, 6 tablespoons unsweetened cocoa powder
In a small bowl, whisk together the sugar, flour, baking soda, and salt. Add the dry mixture into the wet mixture, mixing until well combined. Add in the eggs and sour cream, mixing in between each addition. Pour the batter into the prepared baking sheet and bake for 20 minutes.
2 cups granulated sugar, 2 cups all-purpose flour, 1 teaspoon baking soda, ½ teaspoon kosher salt, 3 large eggs, ½ cup sour cream
In a small saucepan set over medium heat, melt the butter on the stove. Add in the cocoa powder and milk, heating just until boiling. Remove from the stove. Pour the mixture into a bowl and beat in the powdered sugar and vanilla. Once mixed, pour directly onto the hot cake and it will set up. DO NOT let the frosting mixture sit for too long as it will harden in the pan and won’t spread onto the cake. Sprinkle with walnuts or sprinkles, if desired. Allow the icing to set and completely cool before covering/storing.
½ cup unsalted butter, 4 tablespoons unsweetened cocoa powder, 6 tablespoons milk, 3 cups powdered sugar, 1 teaspoon pure vanilla extract, 1 cup chopped walnuts or sprinkles