Set a heatproof bowl over a pan of simmering water, ensuring the base of the bowl does not touch the water. Add the chocolate and melt until smooth, then remove from heat to cool slightly.

In a chilled mixing bowl, whisk the double cream and vanilla extract until soft peaks form.

In a separate, clean, grease-free bowl, whisk the egg whites with a pinch of salt until stiff peaks form.

Gradually whisk the caster sugar into the egg whites until the mixture is glossy and holds its shape.

Gently fold the cooled melted chocolate into the whipped cream using a metal spoon or spatula until just combined.

Add one-third of the whisked egg whites to the chocolate mixture and fold gently to lighten the base.

Carefully fold in the remaining egg whites in two batches, being cautious not to deflate the aeration, until no white streaks remain.

Spoon the mixture into four individual ramekins or serving glasses.

Refrigerate for a minimum of 2 hours to allow the mousse to set before serving.