Ingredients

for the cake:
1. 2 c all-purpose flour
2. 2 c granulated sugar
3. 1 tsp baking soda
4. 1/4 tsp salt
5. 1 c (2 sticks) unsalted butter
6. 1/3 c + 2 Tbsp unsweetened cocoa powder
7. 1 c water
8. 2 eggs
9. 1/2 c buttermilk or sour milk (see note)
10. 1 1/2 tsp vanilla extract
for the frosting:
11. 1/4 c unsalted butter
12. 3 Tbsp unsweetened cocoa powder
13. 2 Tbsp semi-sweet chocolate chips
14. 1/2 tsp vanilla extract
15. 3 Tbsp buttermilk or sour milk (see note)
16. 2 to 2 1/4 c powdered sugar

Instructions

1. Grease a 15"x10" sheet pan. Set aside.
2. Preheat the oven to 350 degrees F.
3. In a medium bowl whisk together the dry ingredients (flour, sugar, baking soda and salt). Set aside.
4. Place butter, cocoa powder and water in a medium saucepan and cook until starts to bubble. Stir constantly. Remove from the heat.
5. Pour the cocoa butter mixture into a large mixing bowl of a stand mixer, and mix on low speed for 2 to 3 minutes. It will be hot so be careful!
6. Increase the speed to medium and add eggs, buttermilk and vanilla one at a time. Beat well after each addition. Beat the batter for about 1 minute. It will be thin.
7. Pour the batter into prepared pan.
8. Bake 25 to 30 minutes or until the toothpick inserted in the center of the cake comes out clean.
9. Cool completely in the pan.
for the frosting:
10. In a medium saucepan, combine all the ingredients except vanilla and powdered sugar. Cook until it starts to boil. Remove from heat.
11. Pour into a mixing bowl. Sift the powdered sugar over the cocoa butter mixture. With a whisk, whisk it in until smooth. Whisk in vanilla.
12. Pour the frosting over cooled cake. Spread evenly. Let sit for at least 30 minutes until the frosting sets.

Notes