Make a double boiler and bring the water to a soft simmer. Put the chocolate in a heat resistant bowl and leave to melt. Put a mixing bowl in the freezer.
Split the eggs and whip the egg yolk with the sugar till it becomes light and creamy. Add the melted chocolate, rum and espresso.
Place the cream in the bowl that came out of the freezer and whip until it forms firm peaks. Add the cream to the chocolate mixture and fold it in with a spatula until combined.
Mix the egg whites until it forms firm peaks and add that to the chocolate mixture as well. Fold it in carefully.
Divide the mixture over 6 glasses and put in the fridge overnight. Serve with extra cream if you want.