Preheat oven to 375°F. Butter a 16x10x1 inch baking pan (or Baker’s half sheet pan).
Mix flour and sugar in a large mixing bowl; set aside.
In a saucepan, combine butter, cocoa, and water; bring to a boil and take off heat.
Stir the butter/cocoa mixture into the flour/sugar mixture. Add sour cream, eggs, vanilla, baking soda and salt. Mix well.
Pour into prepared pan. Bake 15-20 minutes or until cake tests done.
In a saucepan, combine butter, cocoa, and milk. Bring to a boil and remove from heat.
In a mixing bowl, combine powdered sugar and vanilla.
Add hot butter/cocoa mixture to sugar mixture; beat well.
Note: Spread icing over cake as soon as it comes out of the oven.