Break or chop the chocolate up into small pieces and add them together with the brown sugar to a large mixing bowl. Pour in the milk and place the bowl over a small saucepan filled with water. Place the pan over medium heat and start melting the chocolate bain marie.
Transfer the egg whites to a high cup. Beat the egg whites for a couple of minutes until stiff.
Once the chocolate is fully melted, take the bowl off the heat and pour in the cream and the beaten egg yolks.
Carefully pour or scoop this into cold glasses or cups. Place the chocolate mousse in the fridge and let it set and chill for at least 5 hours. Serve chilled.