Collect the egg yolk in bowl or a saucepan suitable for a bain-marie. Add sugar and beer to the yolks. Whisk the ingredients until thoroughly combined, then put them in a double boiler.
It is important not to heat the yolks abruptly, as you might find yourself with a prosaic batch of scrambled eggs. Keep whisking the yolks over slowly simmering water until you'll get a thick and smooth zabaione. It will require from 10 to 12 minutes.
Move the zabaione into a small bowl and place it into a larger bowl filled with cold water and ice cubes. Keep stirring until it is cold.
Now beat in the mascarpone and mix thoroughly, until you have removed any lump.
Whip the cream and fold it gently into the mascarpone.
Take 8 small glasses. Put on the bottom of each glass a savoiardo, quickly dunked into the beer. Cover with a layer of mascarpone, then add some chopped dark chocolate, then another layer of soaked savoiardi, a layer of mascarpone, a third layer of soaked savoiardi and finished with a layer of mascarpone.
Dust with cocoa powder and sprinkle with dark chocolate, then stash in the fridge for at least an hour before serving.