Place the chocolate and water in a heatproof bowl set over a pot of simmering water (double boiler). Stir gently until the chocolate is completely melted and glossy.

Remove from heat, stir in the vanilla extract, and let the mixture cool to room temperature.

In a chilled mixing bowl, whip the heavy cream and powdered sugar on medium-high speed until soft, airy peaks form.

Gently fold one-third of the whipped cream into the cooled chocolate to lighten it, then carefully fold in the remaining cream until uniform in color.

Divide the mixture into individual serving glasses and refrigerate for at least 2 hours before serving to allow the mousse to set.