Preheat the oven to 350 degrees Fahrenheit and line 24 muffin cups with paper liners. Prepare the yellow cake mix as directed on the package.
In a small bowl, combine the instant coffee and 2 teaspoons of hot water. Stir until the coffee is fully dissolved. Add it to the cake batter along with the 4 ounces of chopped chocolate. Stir to combine.
Divide the batter evenly among the muffin cups. Bake for 18 minutes or until a toothpick inserted into the center of the cupcake comes out clean. Let cupcakes cool for 5 minutes before transferring to a wire rack to cool completely.
Set a large bowl over (not in) a pot of barely simmering water. Add sugar, egg whites, and cream of tartar to the bowl and whisk constantly until the mixture is frothy and the sugar is dissolved, about 4 minutes.
Remove the bowl from the heat and transfer to a stand mixer. Beat on medium until soft peaks form, about 4 minutes. Increase the speed to high and beat until the whites form medium-stiff, glossy peaks, about 3 minutes.
Transfer the frosting to a piping bag or large Ziplock bag with the tip cut off. Frost the cupcakes and sprinkle finely grated chocolate on top.
Notes
If you don't drink coffee, pero makes a great substitute!