Preheat the oven to 350 degrees and prepare a 9 by 9 inch baking dish by spraying with nonstick cooking spray.
In a large bowl, mix the flour, sugar, salt, baking soda, and cocoa powder. Set aside.
Measure the water, vegetable oil, vanilla, and vinegar in a 2 cup liquid measuring cup. Whisk for 1 minute.
Stir into the dry ingredients quickly and then immediately pour into prepared pan.
Bake for 30-35 minutes, until it passes the toothpick test. Allow to cool before frosting.
To create the frosting, add the butter to a small bowl and whip until fluffy using a fork.
Add powdered sugar, cocoa powder, and pinch of salt.
Mix well, incorporating the butter into the dry ingredients. Add milk and vanilla and continue to whip until smooth.
Frost cooled cake. Sprinkle with course salt, if desired.