Whip the cream until stiff and cover and chill. Break the whole chocolate into pieces and melt in a hot water bath. Remove the bowl from the water bath. Let the espresso cool down a little.
Separate eggs. Whisk the egg yolks and 1 tbsp. sugar in a hot water bath until thick and creamy. Remove the bowl from the water bath. Stir in chocolate, espresso and approx. 1⁄3 cream. Let the cream cool down.
Beat the egg whites until stiff, adding 1 tablespoon of sugar. Fold the remaining cream first, then the beaten egg white into the cream. Pour the mousse into glasses and chill for at least 3 hours.
Heat the milk just before serving, but do not bring to the boil. Froth and spread on the espresso mousse. Decorate with mocha beans and serve immediately.