Chocolate Sheet Cake: Preheat your oven to 350 degrees F (180 degrees C) and place the oven rack in the center of the oven. Butter, or spray with a non stick vegetable spray, a 9 x 13 inch (23 x 33 cm) cake pan. Then line the bottom of the pan with parchment paper. 

In the bowl of your electric stand mixer, fitted with the paddle attachment, (or with a hand mixer) beat the sugar, flour, cocoa powder, baking soda, baking powder, and salt until combined.

Next, in a large bowl, whisk together the eggs, buttermilk, oil, and vanilla extract. On low speed, add 1/3 of the wet ingredients to the dry ingredients and beat until the mixture is a thick smooth paste. Add another 1/3 of the wet ingredients and mix, on low speed, until the batter is nice and smooth. At this point, scrape down the sides and bottom of your bowl. Finally, add the rest of the wet ingredients and beat until incorporated and you have a smooth batter (the batter will be fairly thin).

Pour the batter into the pan and bake for about 27 to 30 minutes or until a toothpick inserted into the center of the cake comes out clean and the cake is just starting to pull away from the sides of the pan. Remove from oven and let cool completely on a wire rack before frosting.

Chocolate Frosting: Melt the chocolate in a heatproof bowl placed over a saucepan of simmering water. Remove from heat and let cool to room temperature.

In the bowl of your electric stand mixer, fitted with the paddle attachment (or with a hand mixer), beat the butter until smooth and creamy (about 1 minute). Add the sugar and beat until it is light and fluffy (about 2 minutes). Beat in the vanilla extract. Add the melted chocolate and beat on low speed until incorporated. Increase the speed to medium-high and beat until frosting is smooth and glossy (about 2 - 3 minutes). Spread the frosting on the top of the cake with an offset spatula or the back of a spoon. If desired, garnish the cake with chocolate or candy sprinkles, shaved chocolate, or chopped nuts.

Makes 18 - 24 servings.

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