Preheat the oven to 350ºF and prepare three 6-inch or two 8-inch cake pans by spraying the sides with cooking spray and fitting the bottoms with a wax paper or parchment cake circle.
Add about an inch of water into a medium saucepan and bring it to a simmer. Place the chocolate and hot coffee, or water, into a heat safe bowl and place it on top of the simmering saucepan, making sure the bottom of the bowl does not touch the water in the saucepan. Whisk the chopped chocolate mixture constantly until the chocolate is fully melted and the mixture is homogenous. Set aside.
In a medium bowl, whisk together the all purpose flour, cocoa powder, baking powder, baking soda, and salt. Set aside.
In the bowl of your stand mixer (a handheld mixer works fine too!), beat the butter for on high for two minutes until it's light and creamy. Add 1 1/4 Cups (250g) of the sugar (set aside the remaining 1/4 Cup) and continue to mix on high for another two minutes, scraping down the bowl and paddle as needed.
Turn the mixer to low and add the eggs yolks one at a time. Add the vanilla and the melted chocolate and beat until incorporated. Turn the mixer back to low and add the flour mixture and buttermilk in three additions, beginning and ending with the flour mixture. Mix only until the ingredients start to come together.
In a separate bowl with a whisk attachment, add the reserved 1/4 Cup (50g) of sugar and the egg whites. Beat on high speed for 2 minutes, until soft peaks form. Fold about one Cup of the whipped egg whites into the batter to lighten it, then carefully fold the remaining the egg whites into the batter.
Divide the batter evenly among the prepared pans (be sure to fill the pans no more than 2/3 full) and smooth the tops. Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean and the tops spring back when lightly pressed. Let the cakes cool in the pans for about 5 minutes before turning them out onto a wire rack or baking sheet to cool for a few more hours. Make sure they're entirely room temperature before applying any frosting.