Mix together flour, sugar, salt, in a bowl and set aside
In a sauce pan, add water, cocoa and butter. Stir until butter melts and mixture comes to a boil. Pour over dry ingredients and stir.
Add in eggs, buttermilk, baking soda, vanilla and vinegar. Mix to combine.
Pour into a greased 15x10x1 pan
Bake at 350 for 25 minutes
Frosting:
Mix butter, milk and cocoa together
Add powdered sugar and vanilla
Beat well and spread on hot cake as it comes from oven
Video
Notes
Recipe make one 15x10 inch sheet cake.
Use a plastic disposable knife to cut the cake into pieces. Using a plastic knife helps you get a nice clean cut, the frosting doesn't build up on the knife.