Heat oven to 350 degrees. You'll need a 12x16 or 18 inch pan.
In a large bowl, combine the flour, sugar and salt. Stir together and set aside in another bowl, mix the buttermilk, eggs, vanilla and baking soda. Mix with a fork and set aside.
In a medium saucepan melt the butter and cocoa. Whisk together to combine. Meanwhile, bring 1 c water to a boil.
Pour boiling water into the pan. Allow to bubble for a moment, then turn off the heat.
Pour the chocolate mixture into the flour mixture. Stir together for a moment to col the chocolate, then pour in the egg mixture.