Preheat oven to 350 degrees and line 24 muffin cups with paper liners. Prepare yellow cake mix as directed on package. In a small bowl, combine instant coffee and 2 teaspoons hot water. Stir until coffee is fully dissolved and add to the cake batter along with the 4 ounces chopped chocolate. Stir to combine.
Divide batter among muffin cups and bake until a toothpick inserted into the center of a cupcake comes out clean, about 18 minutes. Let cupcakes cool in pans 5 minutes, then transfer 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 and whisk constantly until mixture is frothy and sugar is dissolved, about 4 minutes. Remove bowl from heat and transfer to an electric mixer. Beat on medium until soft peaks form, about 4 minutes. Then increase the speed to high and beat until whites form medium-stiff, glossy peaks, about 3 minutes. Transfer frosting to a piping bag or large zip-top bag with the tip cut off. Frost cupcakes and sprinkle finely grated chocolate on top.
If you don't drink coffee, pero makes a great substitute!