Make the Cupcakes: Preheat oven to 350 degrees F. Line a standard-size muffin pan with liners. Place the chopped chocolate and cocoa powder in a medium bowl. Pour the hot coffee over the mixture and whisk until smooth. Refrigerate mixture for 20 minutes.
Meanwhile, whisk together the flour, sugar, salt and baking soda in a medium bowl; set aside.
Whisk the oil, eggs, vinegar and vanilla extract into the cooled chocolate mixture until smooth. Add the flour mixture and whisk until smooth.
Divide the batter evenly between the muffin pan cups. Bake until the cupcakes are set and just firm to the touch, 17 to 19 minutes. Cool the cupcakes in the pan on a wire rack for 10 minutes, then remove the cupcakes from the pan and place on the wire rack to cool completely, about 1 hour.
Make the Frosting: In the bowl of an electric mixer, whisk together the sugar, egg whites, water and cream of tartar. Set the bowl over a pan of barely simmering water and whisk until sugar is dissolved and the mixture reaches 160 degrees F on an instant-read thermometer. Transfer the bowl to a stand mixer fitted with a whisk attachment and beat on high speed until the frosting forms stiff peaks, about 12 to 16 minutes. Add the vanilla extract and beat until combined.
Transfer the frosting to a large pastry bag fitted with a ½-inch round decorating tip (I used an Ateco #808 tip). Pipe a high swirl of frosting onto the top of each cupcake. Transfer the cupcakes to a baking sheet and refrigerate while preparing the chocolate coating.
Make the Chocolate Coating: Combine the chocolate and oil in a medium heat-proof bowl set over a saucepan of barely simmering water and stir until melted and smooth. Transfer to a small (deep) bowl, and cool to room temperature.
Holding each cupcake by its bottom, quickly dip each cupcake into the chocolate to coat the frosting, and place on a wire rack. Let the cupcakes stand at room temperature 15 minutes, then refrigerate for 30 minutes uncovered. Place in a covered container and refrigerate 2 additional hours before serving. Cupcakes can be refrigerated for up to 3 days.