Preheat oven to 225°. Line 1 or 2 baking sheets with parchment paper.
Place egg whites into a large mixing bowl. Add a pinch of salt.
Beat egg whites with a hand mixer or with the wire whip attachment of a stand mixer until stiff. (You want stiff peaks to form. Peaks should remain when you turn the whip upside down.)
Beat in vanilla.
Beat in sugar gradually (about ¼ cup at a time).
Transfer mixture to a piping bag with a large star attachment (optional). Pipe meringue onto baking sheet, making small nests. If you don't want to use a piping bag, you can also use a spoon to transfer the meringue to the baking sheet and make the nests.
Bake for about 45 minutes to 1 hour, or until the outsides are dry (not brown). Turn off the oven and let them cool with the oven door ajar.
Chop the baking chocolate and place in a microwave-safe bowl. Microwave for 30 seconds at a time, stirring with a spatula after each 30 second interval, until the chocolate is fully melted.
Stir in the coffee and vanilla. Set aside and let cool.
Whip the heavy whipping cream using a hand mixer or stand mixer (with wire whip) until it forms a whipped cream consistency.
Stir in the chocolate mixture. Transfer to the refrigerator and chill for at least 2 hours.
Use a piping bag or spoon mousse into the center of each meringue nest.