Preheat oven to 350°F.
In a large bowl or stand mixer, whisk together the dry ingredients - flour, sugar, cocoa, baking soda, baking powder, salt, and powdered buttermilk if you're using the powdered form.
Melt the unsweetened baking chocolate in 20-30 second bursts in the microwave. It's done heating up when the chocolate is almost completely melted, but not all the way. Stir together until all of it is melted and smooth. Set aside.
Whisk together the wet ingredients (sour cream, oil, eggs, vanilla, buttermilk, hot water, and melted chocolate) in a separate bowl. Then combine with the dry ingredients.
Mix together well until combined. Just don't over mix as that can lead to a crumbly cake.
Pour into 3 - 8" greased cake pans, 2 - 9" pans, or 24 cupcake liners. Bake for 22-26 minutes. The cake is done when the toothpick comes out with moist crumbs on it. Transfer to cooling racks when done.