Grind pistachios finely in a food processor until nearly powder; add milk and process to a smooth paste.

In a small saucepan, whisk together egg yolks and sugar until pale. Slowly pour in the pistachio‑milk mixture, stirring constantly over medium‑low heat until it thickens and coats the back of a spoon (~170 °F/77 °C). Do not boil.

Transfer custard to a bowl, cover surface with plastic wrap to prevent skin, and chill until fully cold.

In a separate bowl, whip heavy cream to soft peaks. Gently fold cooled pistachio custard into whipped cream in batches, until uniform mousse texture develops.

Divide mousse into individual serving glasses or a single dish. Chill for at least 2 hours to set.

For ganache, place chopped chocolate in a heatproof bowl. Warm cream just until it simmers, pour over chocolate and let sit for 1 minute, then stir until smooth. Stir in butter if using.

Allow ganache to cool slightly until thickened, then spoon or pour over set pistachio mousse, creating an even layer.

Chill again for another hour to firm the ganache layer. Serve chilled, optionally topped with chopped pistachios or chocolate shavings.