1. Start by grinding the shelled pistachios with powdered sugar in a food processor until finely ground but not pasty. Set aside.
2. In a bowl, whisk the egg yolks with half of the granulated sugar and vanilla extract until pale and creamy, about 3-4 minutes.
3. Gently fold the ground pistachios into the egg yolk mixture, combining thoroughly but carefully.
4. In a separate bowl, beat the egg whites until soft peaks form. Gradually add the remaining granulated sugar and continue beating until stiff, glossy peaks appear.
5. Lightly whip the heavy cream until it holds soft peaks. Be careful not to overwhip.
6. Fold the whipped cream gently into the pistachio mixture, then fold in the egg whites in thirds, preserving as much air as possible for that light mousse texture.
7. Divide the mousse into serving glasses or ramekins. Cover and chill in the refrigerator for at least 3 hours to set.
8. For the ganache, heat the 1/3 cup of heavy cream in a small saucepan until just simmering. Pour over the chopped chocolate and let sit for 2 minutes, then stir until smooth and glossy.
9. Once the mousse has set, spoon or pour the ganache over the top. Chill again for 15-20 minutes to allow the ganache to firm slightly before serving.

A quick tip from my restaurant kitchen days: when folding, use a gentle hand and a slow, deliberate motion to keep the mousse airy and light. That’s the texture that makes this dessert truly special.