Preheat the oven to 350, and spray your bundt pan real good with baking or cooking spray.
In a large mixing bowl, whisk together flour, sugar, cocoa, baking soda, baking powder, and salt.
Add egg, oil, and buttermilk all together and whisk well. Add the hot water and whisk until the batter is smooth, it will be runny - that is ok. Pour into the greased bundt pan. Since this is a runnier batter, lightly tap the pan on the counter - this will bring any bubbles to the top and get rid of them.
Bake for 50 - 55 minutes. The cake will be pulled away from the sides, and a toothpick inserted to the center will come out clean. Remove from the oven and let it sit in the pan for FIVE minutes. After those five minutes, turn the cake onto a cooling BUT don't remove the pan for FIVE more minutes - let it sit essentially upside in the pan for those five minutes and THEN gently pull the pan up and away. Let it cool completely and make the glaze.