Preheat oven to 400°F and grease and flour a sheet cake pan.
Combine flour, sugar, baking soda, and salt in a large mixing bow.
In medium sauce pan, bring water, butter and cocoa to a boil.
Pour cocoa mixture over dry ingredients and mix. Should make a thick paste.
Add sour milk, eggs, and vanilla. Mix well.
Pour batter into prepared pan and bake at 400°F for 15 minutes.
Remove from oven and frost while warm.
Place butter, milk, and cocoa in a medium sauce pan and boil slightly.
Pour into small mixing bowl. Add vanilla and powdered sugar and mix well.
Frost cake while still warm. (Can top with chopped nuts, mini chocolate chips or sprinkles while freshly frosted, if desired.)
*Sour Milk: Start with 1 cup of milk and add 1 Tbsp of either lemon juice or vinegar. Use 3/4 cup of the mixture for the cake and the rest for the frosting.