In a small bowl, sprinkle the gelatin over the water and let stand until softened.
In the bowl of an electric mixer using the wire attachment, beat the cream until softly whipped. A hand mixer will work as well. Place the whipped cream in the refrigerator for 20 minutes.
In a microwave-safe bowl, heat the liqueur until hot, about 45 seconds. Stir in the softened gelatin until dissolved.
Bring a medium saucepan half filled with water to a boil. Place white chocolate in a medium bowl set over the saucepan of simmering water, let it melt, stirring just a few times. Make sure the bowl doesn't touch the boiling water, to avoid overheating. Remove from heat and set aside.
In the bowl of an electric mixer fitted with the wire attachment, beat the egg yolks with the sugar and salt at high speed until pale and thickened, about 5 minutes. Beat in the liqueur gelatin liquid and the melted chocolate.
Using a rubber spatula, manually and gentle fold in the whipped cream in 2 additions. Transfer the mousse to serving bowls, cover with plastic wrap and refrigerate until firm, at least 4 hours or overnight.
Dust with cocoa powder right before serving.
The mousse can be stored in the fridge for one week.