Preheat oven to 350F (see note). Spray two 8" cake pans with nonstick spray (or, if making cupcakes, line muffin pans with cupcake liners).
In a medium heatproof bowl, combine chopped chocolate and cocoa powder. Pout hot coffee over the top (see note) and whisk until smooth. Refrigerate chocolate mixture for 20 minutes.
Combine flour, sugar, salt, and baking soda in a large bowl. Set aside.
After chilling 20 minutes, remove chocolate mixture from the refrigerator. Add oil, eggs, vinegar (see note), vanilla extract, and chocolate extract, whisking until smooth.
Add chocolate mixture to flour mixture and whisk until smooth.
Divide batter between prepared cake pans (see note). Affix wet jackets to the outsides of the pans, if using (see note). Bake 35-38 minutes, or until a toothpick inserted in the center comes out clean. Turn/rotate cake pans halfway through baking to help ensure an even bake.(If making cupcakes, divide batter among 24 muffin cups. Each will be about ¾ full. Bake 15-18 minutes, or until a toothpick inserted in the center comes out clean.)
Cool cakes 10 minutes in the pans, then turn out onto wire racks to cool completely.
Buttercream:
Melt chocolate in microwave on 30-second bursts, stirring in-between bursts, until chocolate is melted and smooth. Set aside to cool.
Place egg whites in the bowl of a stand mixer (see note).
In a medium saucepan, combine sugar and water. Over low heat, stir gently to dissolve the sugar.
Once sugar is dissolved, stop stirring and bring the syrup to a boil over medium-high heat. If sugar begins to crystallize on the sides of the pan, use a wet pastry brush to brush the sides of the pan, dissolving the crystallized sugar.
Once sugar syrup reaches 130F, begin to whip egg whites on medium-high speed using the whisk attachment.
Beat egg whites to soft peaks and boil syrup to 240F (the hope is that the egg whites reach soft peaks and syrup reaches 240F at the same time).
With mixer on low, pour the syrup in with the egg whites in a slow, steady stream (see note), Whip on medium-high speed until the bowl is barely warm to the touch and the meringue is thick and glossy.
Once meringue is thick, glossy, and cooled, gradually add the butter. Allow each addition to completely incorporate before adding the next.
Add vanilla and chocolate extracts and melted chocolate and mix to combine. If buttercream is too soft to properly frost/pipe onto the cake, place the bowl in the refrigerator for a few minutes until the desired consistency is achieved.
Notes
Note on coffee: The cake won't taste like coffee, I swear! The coffee enhances the chocolate flavor, and I highly recommend it! If you would prefer not to, you can use hot water instead.Note on chocolate extract: I highly recommend this (sometimes overlooked) extract! It lends an amazing depth of flavor to chocolate baked goods! If you can't find it in your local grocery store, you can find some [here]. Even better, makes yourself some [Homemade Chocolate Extract] for next time!Note on vinegar: This may seem like a strange ingredient in a cake, but SCIENCE! The vinegar reacts with the baking soda (think second grade volcano science project), which gives your cake a lighter, fluffier texture. You won't taste the vinegar. That said, if you don't have white vinegar, consider taste in what you substitute. Apple cider vinegar would be a good fit, but your aged balsamic would likely lend a strange flavor.Note on oven temp for dark pans: If you have dark pans, reduce oven heat to 325F. Darker pans bake faster, and the lower temperature will help ensure you don't end up with dry cake.Note on weighing batter: To ensure I have the same amount of batter in each pan, I use my kitchen scale (link [here]) and weigh the pans. Each of my pans contained about 1lb 10oz of batter (not including the weight of the pans).Note on cake strips: If you've never heard of cake strips, they're pretty handy. The wet strips affix to the outside of your pan, and help ensure your cake layers bake up with a flatter top, so you cut less cake away when you level the cakes. you can find them [here]. Note on egg whites: Egg whites need to be *completely* free of yolk! Make sure your bowl is squeaky clean, too. Even a drop of fat will keep your egg whites from whipping up properly. If you get yolk in your whites, dump it out, wash the bowl, and start over.Note on handling hot sugar syrup: Don't worry about scraping out the saucepan to get every last bit of syrup. Keep your focus on not burning yourself. Be VERY careful not to touch the syrup, as it will stick to your skin and could cause serious burns!