• Preheat oven to 350℉ and grease a 18"x13" pan (half sheet pan).
• Add water, butter and cocoa powder to a medium saucepan over medium heat. Bring mixture to a boil.
  1 cup water, 1 cup butter, 3 Tablespoons unsweetened cocoa powder
• Meanwhile, in a separate bowl mix together the flour, sugar, baking soda and salt. (use a bowl that is heat safe)
  2 cups all-purpose flour, 2 cups granulated sugar, 1 teaspoon baking soda, ½ teaspoon salt
• In another small bowl mix together the sour cream, eggs and vanilla. Add sour cream/egg mixture to the flour mixture and mix until combined.
  ½ cup sour cream, 2 large eggs, 1 teaspoon vanilla extract
• Once the chocolate mixture in the saucepan is boiling, remove it from heat and pour it into the batter.

  Mix until combined and no lumps remain. I like to use a hand mixer for this.
• Pour mixture into prepared pan and smooth with a spatula to make sure it is in an even layer.

  Bake in preheated oven for 15-17 minutes or until set (mine is usually done around 15 minutes).

  When the cake is a little over half-way through cooking, prepare the frosting.

For the Chocolate Sheet Cake Frosting

• Pre-measure powdered sugar in a heat safe bowl.

  Add milk, cocoa and butter to a saucepan. Bring mixture to a boil. Once boiling, remove it from the heat and stir into the bowl of powdered sugar.

  I like to beat the mixture with my hand-held mixer to get rid of any lumps.

  Pour hot icing over hot cake. Use a spatula to spread it evenly over the cake.

  Allow frosting to set for about 15 minutes before eating.
  6 Tablespoons milk, 3 Tablespoons unsweetened cocoa powder, ½ cup butter, 3 ¾ cups powdered sugar