Combine water and gelatin in a small bowl and whisk until gelatin has completely dissolved. Whisk in cocoa powder and set aside to cool for 15 minutes.
Place the egg whites in a medium bowl and, using an electric mixer, beat until soft peaks form. Add sugar gradually, working with 1-2 tbsp at a time, while you beat the egg whites.
Once the egg white mixture has reached soft peaks, beat in the egg yolks one at a time at high speed. Slowly pour in the gelatin mixture and mix until uniform in color and well-combined. Evenly distribute the mousse into six 8-oz. ramekins or other serving cups (small coffee cups would make a nice presentation) and refrigerate until set, at least 3 hours.
Using a zester, grate dark chocolate over the top of each mousse before serving. Garnish with larger shards of chocolate, if desired.