Chop both chocolates and melt them in a hot water bath. Leave to cool at lukewarm temperature.
Separate eggs. Whisk the egg yolks and 1-2 tbsp. sugar with the whisk of the hand mixer in a hot water bath until thick and creamy. Remove the bowl from the bain-marie. Stir in liquid chocolate and espresso. Let the mixture cool down to room temperature.
Beat the egg whites until stiff, adding 1 tablespoon of sugar. Whip the cream until stiff as well. First fold the beaten egg whites, then the cream into the mocha cream. Pour cream into glasses and chill for at least 3 hours.
Just before serving, heat the milk (do not bring to the boil). Froth the milk and distribute it on the glasses. Decorate with mocha beans and serve immediately.