1. Melt the chocolate in a microwave or over a pan of hot water. Set aside to cool slightly. 2. Bring the milk to the boil in another pan, and then pour this over the melted chocolate. Stir with a whisk. 3. Add the egg yolk and mix thoroughly. Dip your finger in the chocolate to test the temperature - it should be hot but not burning (around 40C). Allow to cool. 4. Whisk the egg whites in a separate mixing bowl, add the sugar a little bit at a time. Whisk briskly, and then add the rest of the egg whites. Stir. 5. Pour the mousse into individual bowls/glasses or into one large serving bowl. 6. Chill for 1 hour before serving.