In a small bowl, sprinkle the gelatin over the cold water and let it bloom for about 5 minutes.

In a separate mixing bowl, whisk together the egg yolks, sugar, and pinch of salt until pale and smooth.

Stir in the vanilla extract to the yolk mixture.

Gently heat the yolk mixture over low–medium heat, stirring constantly, until it thickens slightly and coats the back of a spoon. Remove promptly from heat.

Melt the bloomed gelatin gently (e.g. in short bursts in the microwave) until fully dissolved, and stir it into the warm custard until smooth.

Press plastic wrap directly onto the surface of the custard and chill in the refrigerator for about 1 hour, until cool but not set.

Meanwhile, whip the heavy cream in a chilled bowl to soft‑peak stage.

Once the custard is cooled, gently fold the whipped cream into it in two stages—first half to lighten, then the remainder—to preserve airiness.

Spoon or pipe the mousse into serving cups or dishes, then refrigerate for at least 2 hours (or up to overnight) to fully set.