You will need:
Chop the chocolate finely. Bring the cream up to the boil, turn off the heat, add the chocolate to the cream and stir it around until the chocolate melts in the cream. Add in the alcohol, if using, and whisk in the egg yolks. Whisk the egg whites until just stiff, then stir in a quarter of the egg white, fold in the rest, gently, being careful not to knock all the air out.
Pour the mousse into a glass or cup and pop into the fridge for an hour or two to set. Serve with pouring or softly whipped cream.
NOTE: For a really intense chocolate mousse, leave the two beaten egg whites out and serve in tiny espresso cups. Little amaretti biscuits are also really good here.