In a bowl mix the hot water and the cocoa powder until the cocoa is fully dissolved. Add the rum extract. Set aside to cool until room temperature.
In a large mixing bowl whip the cream until it thickens.
In another bowl beat together mascarpone cheese and sugar until smooth. Using a rubber spatula combine the two mixtures.
Soak the ladyfingers just for a second in the cooled cocoa liquid. Arrange them on the bottom of a big tin close one to another. Optionally sprinkle some chocolate shaves or sprinkles on top. Cover the layer of ladyfingers with a layer of mascarpone filling. Continue the same process with another layer of ladyfingers, chocolate shaves (optionally) and filling.
Cover the tin with cling film and refrigerate for 8 hours or overnight. Sprinkle cocoa powder on top before serving.
Powdered sugar can be substituted by xylitol which is beneficial for the teeth and would not change the taste of the dessert.
Soaking: Many people consider that the ladyfingers should be soaked only on one side. Other recipes suggest soaking from both sides, just for a second. Soaking from both sides ensures that the dessert is very moist, which in my opinion is crucial for a great tiramisu. The downside of this method is that when you serve it, you will need to spoon it as it will be hard to make good looking slices. So my advice is: if you prepare the tiramisu in individual cups or you don’t care how you will present it when served, soak the ladyfingers on both sides. If you want to serve the Tiramisu in neat pieces, soak the ladyfingers on one side only.
Tin size: The listed quantities of ingredients would fit into tin sizes 8 x 12 inches (20 x 30 cm) or 9 x 12 inches (23 x 30 cm). If you don’t have such tins, don’t worry: you can make the tiramisu in almost any dish, even in a plate. You just need to arrange neatly the soaked ladyfingers and cover them with the mascarpone filling. Another option would be to make individual portions in glasses. I used a smaller size tin and the rest of the ingredients I layered into a small baking dish.
Original Tiramisu recipe: If you want to make authentic Tiramisu, use the same quantity of hot strong coffee instead of hot cocoa and substitute the cream with 4 divided eggs. Instead of the rum extract you can add 2-3 tbs marsala wine, but it is not mandatory for an authentic taste. To make the filling beat the egg yolks with the sugar until creamy. Little by little add the mascarpone and beat until smooth. In a separate bowl beat the egg whites until soft peaks form. Fold the egg whites into the mascarpone mixture. The process of soaking the ladyfingers into the cooled coffee (with or without marsala) and layering the tiramisu is the same as in the cocoa recipe. For an authentic Tiramisu skip the chocolate between the layers, just sprinkle with cocoa powder on top.