In a small bowl, sprinkle the unflavored gelatin over the cold water. Let it sit for 5-10 minutes until it has absorbed the water and become spongy.

Place the finely chopped chocolate in a medium heat-proof bowl. Set it over a saucepan of barely simmering water (a double boiler) and stir until just melted and smooth. Remove from heat and set aside to cool slightly.

In a separate bowl, whisk the egg yolks and granulated sugar together until pale and slightly thickened. While whisking constantly, slowly stream in the hot water to temper the eggs.

Transfer the egg mixture to a small saucepan and cook over medium-low heat, stirring constantly, until it thickens enough to coat the back of a spoon (about 2-3 minutes). Do not let it boil. Immediately remove from heat and stir in the bloomed gelatin until it's completely dissolved.

Pour the hot custard over the melted chocolate and stir until fully combined and glossy. Stir in the vanilla extract. Let this mixture cool to room temperature.

In a large bowl, use an electric mixer to beat the very cold heavy cream until stiff peaks form. Be careful not to over-whip.

Gently fold about one-third of the whipped cream into the chocolate base to lighten it. Then, carefully fold the lightened chocolate mixture into the remaining whipped cream until just combined and no streaks remain. Do not stir, as this will deflate the mousse.

Cover the bowl with plastic wrap and refrigerate for at least 4 hours, or until the mousse is firm and set.