In a large mixing bowl, use an electric mixer to whip the heavy cream and vanilla extract until stiff peaks form, about 3-5 minutes.

Gently fold in the four large egg yolks using a spatula.

Combine dark cooking chocolate and unsalted butter in a microwave-safe bowl, heating in short bursts until melted, about 1-2 minutes.

Pour the melted chocolate mixture into the whipped cream and egg yolk fold, and blend until smooth.

In a separate bowl, whip the four egg whites, gradually adding caster sugar until stiff peaks form, about 3-4 minutes.

Gently fold the whipped egg whites into the chocolate mixture until just combined.

Spoon the mousse into individual cups, cover, and refrigerate overnight or for at least 4 hours.

Serve topped with dollops of crème fraiche and chocolate curls.