Place chocolate in large bowl. Cover with hot coffee. Let sit 4 to 5 minutes, then whisk until chocolate is smooth. If you plan to use an electric mixer, do this in the bowl of the mixer.
Sift together flour, cocoa, baking soda, and baking powder in second large bowl.
Into melted chocolate, whisk sugar, salt, oil, buttermilk, and vanilla until combined. This can be done by hand or in a electronic mixer.
Whisk in eggs, one at a time, by hand or with mixer.
Add flour mixture to chocolate. Whisk until smooth.