Preheat over to 350 degrees F and grease 18x13 inch pan.
Add water, butter, cocoa powder to a medium saucepan over medium heat. Bring to boil.
In a seperate bowl mix together the flour, sugar, baking soda, and salt while sauce is heating up.
In another small bowl mix together the sour cream, eggs, and vanilla.
Combine sour cream/egg mixture and the flour mixture and mix.
Once sauce is boiling, remove from heat and pour it into the batter, mix until no lumps remain.
Pour mixture into prepared pan and smooth with spatula
Bake for 15-20 minutes or until set.
During last 10 minutes of baking, prepare frosting.
Add milk, cocoa, and butter to saucepan. Bring to boil.
Once boiling, remove sauce from heat and add powdered sugar, mix until no lumps.
Pour hot frosting over cake, spread with spatula, let cool before eating.