This is my go to recipe if I need to feed a crowd and I want them to LOVE it. It's also known as Texas Sheet Cake. 

CAKE

Ingredients:

• 2 cups all-purpose flour
• 2 cups sugar
• 1 teaspoon baking soda
• 1/4 teaspoon salt
• 1 cup butter
• 1/3 cup unsweetened cocoa powder
• 2 eggs
• 1/2 cup buttermilk or sour milk
• 1 1/2 teaspoons vanilla

Preparation:

Grease and flour a 15X10X1-inch or jelly roll pan or a 13X9X2-inch baking pan; set aside. In a large bowl, combine flour, sugar, baking soda, and salt; set aside. In a medium saucepan combine 1 cup butter, 1/3 cup cocoa, and 1 cup of water. Bring mixture to a boil, stirring constantly. Remove from heat. With an electric hand-held mixer on medium speed, beat chocolate mixture into the dry mixture until thoroughly blended. Add eggs, 1/2 cup of buttermilk, and 1 1/2 teaspoons of vanilla. Beat for 1 minute (batter will be thin). Pour batter into the prepared pan.
Bake in a 350° oven about 22 to 25 minutes for the 15X10-inch pan or about 30 to 35 minutes for the 13X9-inch pan, or until a wooden pick or cake tester inserted in center comes out clean.
Pour warm chocolate frosting over the warm cake, spreading evenly. Place cake in pan on a wire rack; cool thoroughly before cutting.
Makes 24 servings.

Frosting

Ingredients: 

1/2 cup butter, softened
2 2/3 to 3 1/2 cups of confectioners' sugar
1/3 to 1/2 cup half and half cream or unwhipped whipping cream.
1 tsp Vanilla
1/2 cup unsweetened cocoa powder (add more for dark chocolate flavor)

Directions:
Beat butter for a few minutes.  Add the rest of the ingredients.  Mix till it's the consistency you'd like it.  It you want it thicker add more powdered sugar.  If you want thinner frosting add more whipping cream.