Whisk the egg whites to soft peak. Don't overwhisk or you will get runny eggwhites!
Melt the chocolate over a bain-marie or in 10 second bursts in the microwave. If you are using butter and/or sugar, add these before you start the melting process. Once the chocolate is completely liquid, stir in the alcohol or any other flavouriing you choose (coffee or orange zest for example). Then, quickly stir in the egg yolks and beat well till smooth.
Carefully fold the chocolate mixture into the egg whites using a metal spoon or round ended knife. Don't worry too much if the mixture becomes considerably denser, the volume will probably reduce to around two-thirds or a half of the original whisked egg whites.
Pour the mixture into ramekins or dessert bowls for serving and put in the fridge to set for 30 minutes. The chocolate mousse will be fine for up to 24 hours, though you may want to cover the dishes with cling film to prevent cross contamination
Serve with a dollop of cream, creme fraiche or mascarpone to taste