Bloom gelatin in cold water and set aside. In a food processor, blend shelled pistachios with sugar until a fine paste forms.

Whisk egg yolks over a double boiler until pale and thickened, then gradually whisk in the pistachio paste, vanilla extract, and salt.

Gently heat the bloomed gelatin until dissolved, then stir it into the pistachio-egg yolk mixture and allow it to cool slightly.

In a separate bowl, whip the chilled heavy cream until soft peaks form, then gently fold the whipped cream into the cooled pistachio mixture until well combined.

Divide the mousse into serving glasses or a large bowl and refrigerate for at least 3 hours, or until fully set.

Place chopped dark chocolate in a heatproof bowl. Heat heavy cream in a small saucepan until simmering, then pour it over the chocolate.

Let the mixture sit for 5 minutes, then whisk until smooth and glossy. Allow the ganache to cool to room temperature, stirring occasionally, until it slightly thickens.

Pour the cooled ganache over the chilled pistachio mousse and return to the refrigerator for an additional 30 minutes to set.

Garnish with chopped pistachios and chocolate shavings just before serving for an elegant finish.