Soak gelatine in cold water (if using leaves).
In a bowl, whisk egg yolks with sugar until pale.
Heat pumpkin purée gently, then add softened gelatine (or dissolved powdered gelatine). Stir until smooth.
Add vanilla, cinnamon, nutmeg, and a pinch of salt. Let cool slightly.
Fold in whipped cream until airy. Spoon into glasses or molds. Chill for at least 3 hours.
In a saucepan, combine sugar and water, bring to a boil.
Add orange zest strips and simmer until translucent, about 15–20 minutes.
Drain and cool on parchment paper.
Pulse toasted hazelnuts, sugar, melted butter, and salt in a food processor until crumbly, resembling soil.
Spread on a baking tray and bake at 170 °C (340 °F) for 10 minutes, until lightly golden. Cool completely.
Spoon hazelnut soil onto plates or over mousse glasses.
Garnish mousse with candied orange strips.
Serve chilled with extra toasted hazelnuts if desired.
