In a large bowl, using an electric mixer fitted with a whisk attachment, beat the egg yolks and powdered sugar on medium-high speed until creamy and the sugar is completely dissolved, 2 to 3 minutes.
Add the mascarpone and mix on medium speed just until smooth and creamy, about 1 minute.
Quickly dip both sides of the ladyfingers into the coffee, 1 to 2 seconds per side, and arrange 9 to 12 ladyfingers on the bottom of a 9 x 7 x 3–inch (23 x 18 x 8 cm) or 8 x 8 x 3–inch (20 x 20 x 8 cm) casserole dish. The necessary amount of ladyfingers will depend on the size of the ladyfingers as well as that of the casserole dish. The required amount of coffee depends on how long you dip the ladyfingers and how much they soak up. I don’t recommend soaking the ladyfingers for too long; otherwise, the Tiramisu will end up very watery.
Spread about one-third of the mascarpone cream on top of the ladyfingers. Repeat twice more, so that the Tiramisu is assembled as follows from bottom to top: ladyfingers + cream, ladyfingers + cream, ladyfingers + cream.
Wrap the casserole dish tightly with plastic wrap so that no air or odors from the fridge can get inside the tiramisu. Refrigerate overnight for about 8-12 hours. Before serving, dust the top lightly with cocoa powder. Store in the fridge for up to 3 days.