Combine the egg yolks and sugar. Beat well then set over a simmering pot of water and continue to beat until the mixture is too hot to dip your finger into. Remove from the heat and continue to beat for another five minutes.
In a separate heat-proof bowl combine the chocolate, espresso, and brandy. Set over the simmering water until the chocolate has melted. Mix the chocoalte, coffee, and brandy until they are smooth then mix into the egg yolk and sugar mixture.
Beat the egg whites on high with a pinch of salt for five minutes or until they reach stiff peaks. Fold the whites into the yolk and chocolate mixture.
Spoon the mousse into ramekins, cover with plastic, and cool in the fridge for two hours before serving or use to make a chocolate mouse layer cake.