Chocolate Sheet Cake with Chocolate Buttercream

Preheat the oven to 350ºF (177ºC). Grease and flour (or use homemade cake release) a 9" x 13" cake pan. Set aside.
In a large bowl, whisk together the granulated sugar, brown sugar, flour, cocoa powder, baking powder, baking soda, and salt. Set aside.
1 cup (200g) granulated sugar, 1 cup (200g) firmly packed light brown sugar, 2 cups (240g) all-purpose flour, ¾ cup (72g) Dutch processed cocoa powder, 2 teaspoons baking powder, ½ teaspoon baking soda, 1 teaspoon salt
In a small bowl, whisk together the eggs, vegetable oil, and vanilla extract until completely combined.
4 large eggs, ½ cup (113mL) vegetable oil1, 2 teaspoons vanilla extract
Pour the wet ingredients into the dry ingredients, whisk until the batter starts to come together, then add the coffee, and whisk again until batter is smooth.
1 and ¼ cup (300mL) hot coffee2
Pour the batter into the prepared pan. Bake for 18-20 minutes, or until a toothpick inserted into the center of the cake comes out mostly clean. Remove from oven and allow cake to cool in the pan completely on a wire rack before adding the frosting.

In a medium size bowl, whisk together the powdered sugar, cocoa powder, and salt. Set aside.
2 cups (240g) powdered sugar, 6 Tablespoons (36g) Dutch processed cocoa powder
In a large bowl with a handheld mixer or in the bowl of stand mixer fitted with the paddle attachment, beat butter on medium speed until smooth (about 2-3 minutes).
⅙ teasponn salt, ½ cup (113g) unsalted butter, 1 and ½ teaspoons vanilla extract
Decrease mixer speed to low, then add the vanilla extract and 2 Tablespoons of the milk or cream.
2 to 3 Tablespoons (30-45mL) milk or cream
Slowly add the powdered sugar/cocoa powder mixture and continue to mix on low until a cohesive frosting starts to form. Increase the mixer speed to medium-high and add more milk or cream as needed, 1 to 2 teaspoons at a time until desired consistency.
Use an offset spatula to spread the frosting over the top of the cake, then slice and serve. Cake stays fresh covered at room temperature for up to 4 days. Cake may be covered and refrigerated for up to 7 days. Unfrosted cake may be frozen, wrapped tightly, up to 2 months. Thaw overnight in the refrigerator before frosting. Frosted cake can be frozen, wrapped tightly, up to 2 months. Thaw overnight in the refrigerator and bring to room temperature before serving.