Preheat the oven to 350°. Lightly butter and flour a 9-x-13-inch baking pan, 2, 9-inch round cake pans or a 12-inch Dutch oven.
In a large bowl, whisk together the flour, sugar, and baking soda.
In a small saucepan combine the water, butter and cocoa. Bring to a boil over medium-high heat, stirring constantly. Let boil for 30 seconds.
Slowly whisk the cocoa mixture into the flour mixture. Whisk in the buttermilk, eggs and vanilla.
Pour the batter into the pan(s). Bake for about 25 minutes or until an inserted toothpick comes out clean.
Allow to cool to room temperature before removing from the pan and icing with the Buttercream icing (below)
