Prepare the coffee, add the rum (optional) and let it cool down completely.
Coarsely chop the chocolate, and set aside.
Divide the egg yolks from the whites: put the yolks in the bowl of your stand mixer (KitchenAid) and start beating them.
Prepare the syrup for pasteurizing the eggs. Bring the water and sugar to 250°F (121°C). Use a food thermometer. If you do not have one, just look at the sugar. If it makes bigger and thicker bubbles than before, it’s ready.
Gently pour in the sugar syrup into the egg yolks and beat them on medium speed until the mixture cools down to about 68°F (20°C) where they are fluffy and pale yellow.
Stir the mascarpone until creamy and fluffy; in the beginning, it will be quite sticky, and then it will get runnier.
Add the vanilla seeds or extract to the mascarpone cream and mix well with a wooden spoon.
Dilute the mascarpone with a little egg yolk mixture, which will get softer. Incorporate the remaining part of the egg mixture with gentle downwards movements.
Whip the egg whites or cream into stiff peaks and fold in the mascarpone cream. Here again, use a little whipped whites to dilute the mascarpone cream.
Then add the remaining with gentle downward movements.
Now to assemble your dessert: dip the ladyfingers on both sides in the cold coffee.
Arrange them in a glass or aluminum pan or ceramic dish.
Spread the mascarpone cream on the top; sprinkle with cocoa powder and the coarsely chopped chocolate.
Repeat one more time, spread the last layer of cream, and top with cocoa powder and chocolate shavings.
Wrap with plastic wrap and refrigerate overnight.
Top it with more cocoa powder just before serving.
Tiramisu doesn’t last long: it stays fresh up to 3 days in the fridge.