Chocolate Sheet Cake

Ingredients

• CHOCOLATE CAKE
• 3 cups (360 g) all-purpose flour
• 2 2/3 cups (530 g) granulated sugar
• 1/2 cup (50 g) Dutch-process cocoa powder
• 2 tsp baking soda
• 1 1/2 tsp baking powder
• 3/4 tsp salt
• 1 cup (240 ml) whole or low-fat buttermilk, at room temperature
• 1/2 cup (120 g) sour cream, at room temperature
• 3 large eggs, at room temperature
• 3/4 cup (180 ml) vegetable oil
• 1 tbsp vanilla extract
• 1 1/2 cups (360 ml) hot coffee or boiling water
• CHOCOLATE FROSTING
• 1 1/2 cups (340 g) unsalted butter, at room temperature
• 1/4 cup (25 g) Dutch-process cocoa powder
• 3/4 tsp salt
• 1 lb (450 g) powdered sugar
• 1–3 tbsp whole milk or heavy cream
• 1/3 cup (100 g) hot fudge sauce or chocolate ganache
• Sprinkles, chopped nuts or shaved chocolate, optional

Method

1. Preheat the oven to 350°F (175°C). Lightly grease a 9-by-13-inch cake pan. Line it with a parchment sling if you plan to remove the cake from the pan.
2. Sift the flour, granulated sugar, cocoa powder, baking soda, baking powder and salt into a large bowl.
3. Whisk the dry ingredients together.
4. In a separate bowl, whisk together the buttermilk, sour cream, eggs, vegetable oil and vanilla.
5. Slowly whisk the hot coffee or boiling water into the wet ingredients.
6. Pour the wet mixture into the dry ingredients and whisk until no streaks of flour remain.
7. Pour the thin batter into the prepared pan.
8. Bake for 45–50 minutes, until the centre springs back when lightly pressed and the edges begin to pull away from the pan.
9. Cool the cake completely in the pan.
10. For the frosting, beat the butter until smooth and creamy, about 2 minutes.
11. Mix in the cocoa powder and salt.
12. Gradually add the powdered sugar, about 1/2 cup at a time, scraping down the bowl occasionally.
13. Mix in the hot fudge sauce or ganache.
14. Beat in enough milk or cream to make the frosting smooth, fluffy and spreadable.
15. Spread the frosting over the completely cooled cake.
16. Decorate with optional sprinkles, nuts or shaved chocolate.