Melt the chocolate – Place the chocolate and butter in a heatproof bowl set over a pan of gently simmering water. Stir until smooth, then remove from heat and allow to cool slightly.

Prepare the egg yolks – Stir the egg yolks into the melted chocolate one at a time until fully combined.

Whip the cream – In a separate bowl, whisk the double cream until it forms soft peaks, then set aside.

Whisk the egg whites – Whisk the egg whites until they form soft peaks, then gradually add the caster sugar and continue whisking until glossy.

Fold gently – Fold the whipped cream into the chocolate mixture, followed by the egg whites, using a light hand to keep the mousse airy.

Chill to set – Spoon the mousse into serving glasses or bowls and chill for at least 2 hours until softly set.

Serve – Serve chilled, plain or with a light garnish if desired.