Whip the cream – Using electric beaters, whip the cream with vanilla until soft peaks form. Cover and refrigerate until needed.

Melt the chocolate – Break the chocolate into pieces and place in a heatproof bowl. Set over a pan of barely simmering water (or microwave in short bursts) until smooth. Set aside to cool slightly.

Mix in egg yolks – Lightly beat the egg yolks in a small bowl. Slowly stir them into the cooled chocolate until glossy.

Whip the egg whites – In a clean bowl, use electric beaters to beat the egg whites with a pinch of salt until soft peaks form. Gradually add the sugar, whisking until firm peaks form.

Fold together – Fold one-third of the egg whites into the chocolate to lighten. Gently fold in the remaining egg whites, then the whipped cream. Work slowly to keep the mousse airy.

Chill to set – Spoon the mousse into a large serving bowl or individual little bowls. Cover and refrigerate for at least 4 hours, or preferably overnight, until firm.

Prepare the hazelnuts – Place skinless hazelnuts on a baking tray and roast in a 180°C (fan 160°C) oven for 8–10 minutes, or until golden and fragrant. Roughly chop and sprinkle with a pinch of flaky sea salt while still warm, so the salt clings.

Serve – Sprinkle generously with the salted hazelnuts just before serving.