Beat the sugar and egg yolk in a mixer until it doubles in volume and becomes a light, silky cream.
Add the (cold) mascarpone to the cream. Work with an electric mixer until the cream is smooth and compact. Set aside in a piping bag with a nozzle of your choice.
Moisten the ladyfingers in the (unsweetened) coffee, without wetting them too much. Then cut out the biscuits with a rectangular pastry cutter or a pastry cutter of your choice.
Start the composition with a layer of biscuits, a generous layer of cream, another layer of biscuits and finish with the remaining cream.
Remove the pastry ring, top with cocoa powder and serve.