Separate the eggs, whip the egg whites until stiff, adding 50 g of sugar, a little at a time.
Using a mixer, beat the egg yolks with the rest of the 50 g sugar until light and fluffy. Add spoonfuls of mascarpone and beat for a few minutes until a creamy mixture develops.
Carefully using a spatula, incorporate the egg white into the mascarpone mixture.
Dissolve a tablespoon of sugar in coffee and add the Marsala or Whisky.
Spread a thin layer of mascarpone cream on the base of the casserole dish. Soak the ladyfingers on both sides for a few seconds in the coffee. Layer in the casserole dish, then cover with mascarpone cream. Place another layer of ladyfingers and then cover with the rest of the cream.
Let the Tiramisù rest for a few hours in the refrigerator then sprinkle with the bitter cocoa before serving.