In a small bowl, sprinkle the gelatin over the cold water. Let it sit for 5-10 minutes to bloom.

In a heatproof bowl set over a saucepan of simmering water (or in the microwave at 50% power), melt the chopped chocolate until smooth. Remove from heat and set aside to cool slightly.

In a separate heatproof bowl, whisk the egg yolks with half of the sugar and the salt until pale and slightly thickened. Set this bowl over the saucepan of simmering water and whisk constantly until the mixture reaches 160°F (71°C) on an instant-read thermometer. Remove from heat.

Place the bowl of bloomed gelatin in the microwave for 5-10 seconds, just until it turns to liquid. Immediately whisk the liquid gelatin into the hot egg yolk mixture. Then, whisk the egg mixture into the melted chocolate until fully combined and smooth.

In a large, chilled bowl, use an electric mixer to beat the cold heavy cream until it begins to thicken. Add the remaining sugar and the vanilla extract. Continue beating until stiff peaks form.

Gently fold about one-third of the whipped cream into the chocolate mixture to lighten it. Then, carefully fold in the remaining whipped cream in two additions until just combined and no streaks remain. Be careful not to deflate the mixture.

Cover the bowl with plastic wrap and refrigerate for at least 4 hours, or until firm, before using as a filling.