Chocolate Sheet Cake

My Chocolate Sheet Cake is easy to make and delicious. Everyone in my family loves eating this cake. This cake is chocolaty, fairly sweet, but is perfectly balanced out with the buttermilk's slight tang. The texture is extremely tender and quite moist. I hope you'll give this recipe a try! Serves about 20-24 people.
Ingredients:
2 Cups all-purpose flour
2 Cups granulated sugar
1 teaspoon baking soda
1/4 teaspoon salt
1 Cup (2 sticks) unsalted butter
1/3 Cup unsweetened cocoa powder
1 Cup water
2 Large eggs
1/2 Cup buttermilk, low fat is fine.
1 1/2 teaspoons pure vanilla extract
For Icing:
1 stick of unsalted butter
6 Tablespoons of milk or buttermilk
3 Tablespoons cocoa powder
2 1/4 Cups confectioner's sugar, sifted
1/2 teaspoon vanilla

1. Preheat your oven to 350 degrees F. Adjust an oven rack to the middle position. Grease a baking sheet with sides. Combine the flour, baking soda, and salt in a large bowl. 
2. In a medium saucepan, combine butter, cocoa, and water. Cook on medium-high heat and bring to a boil, stirring occasionally. Remove from heat and slowly beat into the flour mixture. Increase the speed to medium and beat for 2 minutes until combined. Add the eggs, buttermilk, and vanilla and beat for 1 minute.
3. Pour into the greased baking sheet and bake for 20-25 minutes, or until a toothpick inserted in the middle comes out clean.
4. In a medium sauce pan (you can reuse the first saucepan if you want), combine the butter, milk, and cocoa. Bring to a boil. Remove from heat and whisk in the confectioner's sugar and vanilla. If it hardens give it a good whisk.
5. Once cake is done, pour the icing onto the cake and smooth it out. Let cake cool 2 hours until serving.