Method:
To make tiramisu, start with the eggs and separate the egg whites from the yolks.
Whip the yolks with the electric whisk, pouring only half dose of sugar. As soon as the mixture has become clear and frothy, and with the whips still running, you can add mascarpone, little by little. Incorporated all the cheese you will have obtained a thick and compact cream; keep it aside.
Clean the whisk and whip the egg whites by pouring the remaining sugar a little at a time. You will have to whip them like snow; you will get this result when the mixture won't move when you turn the bowl over.
Take a spoonful of egg whites and pour it into the bowl with the yolks and sugar and mix vigorously with a spatula. Then proceed to add the rest of the egg whites, gradually mixing very gently from the bottom up.
Once ready, spread a generous spoonful of cream on the bottom of a 11x7 inches tray. Soak the savoiardi biscuits for a few moments in the cold coffee (sweetened as you like), first on one side and then on the other. Gradually distribute the soaked savoiardi on the cream, all in one direction, to obtain a first layer on which you will distribute another part of the mascarpone cream.
Continue to distribute the biscuits, then make another layer of cream. Sprinkle the last layer with the bitter cocoa powder and leave to harden in the refrigerator for a couple of hours. Your tiramisu is ready!