Preheat oven to 375 degrees F. Line two 12-cup muffin tins with paper liners; set aside.
In the large bowl of a standing mixer, stir together flour, sugar, cocoa, baking soda, and salt. Add eggs, buttermilk, melted butter and vanilla extract and beat until smooth (about 3 minutes). Remove bowl from mixer and stir in hot coffee with a rubber spatula. Batter will be very runny.
Remove bowl from mixer, and pour batter into a large liquid measuring cup with a spout. Divide batter among prepared muffin cups, filling each no more than two-thirds full (or else batter will rise over and make a big mess - not bigger cupcakes). Bake 20-24 minutes, or until cupcakes feel slightly springy when you gently press their centres. Transfer cupcakes to wire racks to cool completely.
Place sugar in a medium saucepan over medium heat. Whisk often as the sugar starts to stick to the bottom, clump up, and then eventually melt. Continue only until the syrup comes to a boil, then stop. You can swirl the pan every now and then, but no more whisk.
Watch very closely as the syrup will go from clear to golden brown very quickly. As soon as it turns amber, remove saucepan from the heat. Working quickly and averting your face, whisk in the cream - holding whisk at arm's length as the caramel will bubble up aggressively. Whisk in butter, salt and vanilla. Set aside to cool. The whole process from sugar to caramel takes only about 5 minutes on my stove.
Combine egg whites, sugar, water, cream of tartar and salt in the top of a double boiler or in a heatproof bowl set overtop of a saucepan of simmering water. Using an electric mixer, beat on high for about 7 minutes - until it is shiny, voluminous and will hold nice peaks.
Remove from heat and beat in vanilla extract. Cool at least 5 minutes before using.
Transfer frosting to pastry bag fitted with a large tip (I used an open star tip). Pipe frosting around the outer circumference of each cupcake.
Use two small spoons (one spoon to scoop, one to scrape off other spoon) to transfer a scant teaspoon of caramel into the centres.
Finish piping frosting onto cupcakes, first retracing your first circle, and moving inwards and up, creating a swirl effect.