This recipe makes huge sheet pans worth, so it's perfect for a holiday party.

This recipe makes a huge sheet pan's worth, so it's perfect for a holiday party. If you don't have a cup of coffee, a cup of warm water should also do the trick. Note that halfway through the cake's baking time, you need to start making the frosting so that it's ready to be poured onto the cake immediately after it comes out the oven. Sprinkle immediately with toasted nuts to get that beautiful look, or, if you're feeling particularly festive, feel free to top off with a sprinkling of crushed peppermints.

Ingredients:
Special Equipment
one 15 by 10-inch baking pan

For the Chocolate Sheet Cake:
(makes one 15 by 10-inch baking pan)
1 cup (2 sticks) unsalted butter
1/2 cup unsweetened, non-Dutch natural cocoa, lightly packed
3 tablespoons vegetable oil
1 cup strongly brewed coffee
2 cups all-purpose flour
2 cups granulated sugar
1 teaspoon baking soda
1/2 teaspoon kosher salt
2 eggs, at room temperature
1/2 cup buttermilk, at room temperature
2 teaspoons vanilla extract

For the Fudge Frosting:
(makes enough for one 15 by 10-inch baking pan)
1/2 cup unsalted butter
1/4 cup Dutch-processed cocoa, lightly packed
1/3 cup whole milk
2 teaspoons vanilla extract
3 cups confectioner's sugar, sifted
1/2 cup toasted chopped walnuts

Recipe:

For the Chocolate Sheet Cake:Center an oven rack and preheat the oven to 375 (F).
Melt 1 cup unsalted butter in a large saucepan over medium heat. Whisk in 1/2 cup cocoa powder. Add 3 tablespoons vegetable oil and 1 cup coffee and bring to a rolling boil for about 30 seconds. Remove the saucepan from heat and set it aside on a cooling rack to cool slightly.
While the saucepan is cooling, whisk together 2 cups flour, 2 cups sugar, 1 teaspoon baking soda, and 1/2 teaspoon kosher salt in a large bowl.
Pour the warm cocoa mixture (from the second step) into the flour/sugar/baking soda/salt mixture and whisk until just combined.
In a small bowl, whisk together 2 eggs, 1/2 cup buttermilk, and 2 teaspoons vanilla.
Using a rubber spatula, stir the buttermilk mixture into the batter until just combined.
Pour the batter into a greased 15 by 10-inch baking pan and place in the center of the oven. Bake for 22 to 25 minutes, or until the top is firmed and a wooden skewer inserted in the middle of the cake comes out with a few moist crumbs.

For the Chocolate Frosting:When the cake is about halfway through its baking time, start making the frosting.
Melt 1/2 cup unsalted butter in a medium saucepan over medium heat. Whisk in 1/4 cup cocoa powder and bring the mixture to a rolling boil and boil for about 30 seconds. Remove from heat and immediately whisk in 1/3 cup milk and 2 teaspoons vanilla extract.
Whisking continuously, add 2 cups confectioner's sugar one cup at a time.
Immediately after the cake comes out of the oven, pour the frosting over the hot cake and sprinkle with 1/2 cup toasted nuts. Try not to jiggle the cake before it sets or you'll leave waves in the frosting.
Cool completely on a cooling rack before cutting into squares. Wrapped well and at room temperature, this cake keeps up to 5 days.

source cake here