Separate the egg yolks from the whites. Beat the yolks with the sugar until the mixture is pale yellow. Whip the egg whites until stiff and incorporate into the egg yolk mixture.
Break up the savoiardi biscuits, distribute them evenly between 4 glasses and soak with the espresso coffee.
Break the chestnuts in half and divide equally between the glasses.
Gradually incorporate the mascarpone into the egg mixture and fill the glasses.
Put the tiramisù in the fridge for about 5 hours. Sprinkle cocoa powder on top before serving.