1. Chop the chocolate into small pieces and place in a heatproof bowl over a bain-marie or a gently simmering water bath. The bowl should not touch the water. Stir occasionally until melted, then remove from heat.

2. Heat the milk until it just comes to a boil, then pour it over the melted chocolate. Whisk slowly until smooth. Add the egg yolk and whisk until fully combined.

3. In a clean bowl, whip the egg whites on medium speed until soft peaks form. Increase speed to medium-high and gradually add the sugar in a slow, steady stream. Continue whipping until the egg whites are firm and glossy.

4. Ensure the chocolate mixture is cooled before adding the egg whites. Gently fold in one-third of the egg whites to lighten the mixture, then carefully fold in the remaining two-thirds until no streaks remain.

5. Pour into individual serving dishes or a large bowl. Chill for about an hour. The mousse will be light and airy if eaten soon after chilling. If left for longer (up to 2 days), it will develop a denser texture.