In a medium microwave-safe bowl, microwave butter until almost completely melted.
Add chocolate and oil; microwave for 30 seconds; stir; microwave for an additional 15 seconds, if necessary, until chocolate is completely melted.
Remove from the microwave, stir until well-combined; set aside to cool.
In a medium bowl, whisk together flour, cocoa powder, baking powder, baking soda and salt; set aside.
In a large bowl, whisk together sugar, eggs and vanilla extract.
Whisk in cooled chocolate mixture. (It can be warm, but if it is hot to the touch, set aside for another couple of minutes; if too hot it will scramble the eggs).
Whisk in half of the flour mixture until just combined.
Add all of the buttermilk and mix well.
Whisk in the last of the flour, mixing until just combined.
Whisk in the coffee or water; the batter will be thin and runny; it's supposed to look like that.
Fill cupcake tins just under ¾ of the way full (tip: if you transfer the batter to a liquid measuring cup first, it makes filling easier); don't overfill; there may be a tiny bit of batter left over.
Place in the pre-heated oven and bake for 15 to 18 minutes, or until a toothpick inserted into the center of the cupcakes comes out clean.
Remove from the oven and place on a wire rack to cool for about 30 minutes.
Take chocolate frosting mixture out of the refrigerator and set on the counter to soften while your cupcakes cool.