Melt Chocolate: Begin by placing dark chocolate in a heat-proof bowl set over simmering water. Stir gently until it’s smooth and completely melted, then allow to cool slightly.

Whisk Egg Whites: Separate the egg whites and yolks. In a large bowl, whisk the egg whites until soft peaks form. Gradually add caster sugar while whisking until the mixture becomes glossy and holds stiff peaks.

Combine Yolks with Chocolate: Carefully stir the egg yolks into the melted chocolate, mixing well. If the chocolate seizes, simply add boiling water, one tablespoon at a time, stirring until it’s glossy again.

Fold in Egg Whites: Mix a third of the whisked egg whites into the chocolate mixture to lighten it. Then, gently fold in the remaining egg whites until no streaks remain, being careful to retain as much air as possible.

Chill & Serve: Spoon the mousse into individual serving dishes, cover, and place in the refrigerator for at least 4 hours—overnight is best—to let it set perfectly.