Whisk the egg yolks with 4 tablespoons sugar in the top of a double boiler over boiling water until thick and creamy about 5 minutes. The egg should be pale and will double in the quantity.
Place the broken chocolate, coffee and rum in the top of a double boiler over boiling water and leave until melted. Make sure the bottom of your bowl doesn't touch the boiling water.
Once melted, stir in cocoa powder. Add butter little by little and stir until fully absorbed. Remove from heat and let it cool to room temperature.
Meanwhile whip egg whites with remaining tablespoon sugar until soft peak form.
Add the chocolate mixture slowly to the egg mixture stir gently until thoroughly incorporated and smooth.
Before you fold egg whites to your chocolate mixture make sure chocolate mixture is room temperature (if too hot it won't set properly and could be too runny).
Fold a quarter of the egg whites into the cooled chocolate mixture, continue with the remaining egg whites using a lift and fold motion with your spatula.
Pour mousse into small ramekins or glasses.
Refrigerate over night.
Serve with berries and chocolate curls if desired.