Preheat oven to 350 degrees F and line a muffin tin with cupcake liners.
In a large bowl, mix flour, sugar, cocoa powder, baking soda, and salt together.
Add the buttermilk, oil, egg, and vanilla and mix until combined. Pour in the boiling water and whisk until smooth. Batter will be very thin.
Fill each cupcake well about 2/3 full (about 1/4 cup of batter) and bake for 15-18 minutes or until toothpick comes out clean or with just a few moist crumbs.
Let cool in pan for 5 minutes and then remove to a cooling rack to cool completely before frosting.
For the Frosting:
With and electric mixer, beat butter and cocoa powder together until smooth. Add the sugar and beat until smooth. Pour in the vanilla and 1 tablespoon of milk and mix for 2 minutes. If still too thick, add more milk to get desired consistency.
For a 9x13 inch cake, double the recipe and bake in a lightly greased 9x13 pan at 275 degrees for 28-30 minutes or until toothpick comes out clean.