In a large saucepan, bring the butter, water and cocoa to a boil. Remove form the heat. Combine the flour, sugar, baking soda and salt; add to cocoa mixture. Stir in the sour cream until smooth. Pour into a Pam sprayed 15 x 10 x 1-inch baking pan. Bake at 350 degrees for 20-25 minutes or until a toothpick inserted near the center comes out clean.
In a saucepan, melt butter; add milk and cocoa. Bring to a boil. Remove from the heat. Whisk in powdered sugar and vanilla until smooth. Pour over warm cake. Cool completely.