Begin by taking the mascarpone out of the fridge at least an hour in advance to soften.
Separate the eggs, placing the yolks in a mixer. Whisk the yolks with 100g of sugar until doubled in size and pale yellow in color.
Fold the egg yolk mixture into the softened mascarpone, mixing lightly until smooth.
Clean the mixer and add the egg whites. Whisk the egg whites with the remaining 100g of sugar until a meringue is formed. Fold the meringue into the mascarpone mixture and add a pinch of sea salt. Set aside at room temperature.
Warm the coffee with the alcohol and pour it into a bowl.
Assemble the tiramisu: Dip the sponge fingers in the coffee-alcohol mixture for about 5 seconds to absorb liquid. Layer these in the base of a rectangular serving dish.
Add an even layer of the mascarpone mixture, spreading it to fill the gaps. Repeat the process until the dish is full. Leave the tiramisu to set in the fridge for at least 3 hours, or preferably overnight.
Melt the dark chocolate and spread it on a marble worktop or a glass chopping board. Allow it to cool in the fridge. Use a scraper or spatula to create swirls in the chocolate.
Decorate the top of the set tiramisu with the chocolate swirls.
Remove the tiramisu from the serving dish, dust the top with cocoa powder, and cut it into squares.
Serve the tiramisu squares on individual plates, garnished with additional chocolate swirls.