Preheat your oven to 375°F. Grease a 9×13 inch pan.
In a large bowl, mix together the flour, sugar, and salt.
In a saucepan, melt the butter. Add cocoa and water, bring to a boil, stirring frequently. Remove from heat, stir in instant coffee, and pour over the flour mixture, beating well.
Add buttermilk, eggs, baking soda, vanilla, and vinegar. Mix well.
Bake for 25-30 minutes or until a toothpick comes out clean from the center. Let it cool while you prepare the frosting.