First make the coffee, add 1 tbsp sugar and dark rum, and let it cool.
Now carefully separate the egg whites from the yolks, remembering that the whites must not contain any traces of egg yolks.
Then whip the egg yolks with a hand mixer, slowly drizzling 4 tbsp of sugar. Once the mixture is clear and fluffy, you can add the mascarpone cheese little by little to prevent grains from forming. When all the cheese is incorporated, you will have a thick and compact cream; set it aside. (Check notes for the double boiler method).
Now, in a new bowl, beat the egg whites until they form stiff peaks. Add 1 tbsp of sugar and 1/2 tsp of cream of tartare and continue whipping until glossy peaks forms. (Check notes for the double boiler method).
Add half of the mascarpone and egg custard cream stirring gently from the bottom to the top to avoid the white egg to collapse. Repeat gently with the remaining cream misture.
Dip the ladyfingers in the cooled coffee for a 1-2 seconds and make a layer in a 13x9 inch baking dish.
Using a spatula, spread the mascarpone-egg cream evenly and continue with another layer of ladyfingers, followed by more cream.
Smooth the top,refrigerate for at least 6 hours to set.
When ready to serve, remove it from the fridge and dust it with the cocoa powder using a fine sieve.
Your tiramisù is ready to be enjoyed!