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.