Soak the pistachio nuts in water for at least 4 hours, or ideally overnight. Drain thoroughly before use.

In a large mixing bowl, whip the heavy cream until soft peaks form, about 3-5 minutes.

Combine the drained pistachios, sugar, and vanilla extract in a food processor and blend into a smooth paste.

Gently fold the whipped cream into the pistachio mixture until well combined.

Pour the mousse into serving glasses and refrigerate for at least 2 hours to set.

Heat the milk in a saucepan until simmering, then pour it over the chopped dark chocolate in a bowl.

Stir until the chocolate melts completely and the mixture is smooth.

Drizzle the warm ganache over the set mousse and serve immediately.