Separate the eggs. In a clean, grease-free bowl, beat the egg whites until stiff peaks form. In a separate bowl, beat the egg yolks with the sugar until pale and fluffy.
Stir the mascarpone until smooth, then add it to the egg yolk mixture. Mix on low speed until combined. Gently fold in the whipped egg whites until fully incorporated. Be careful not to overmix, or the filling will lose its light texture.
Quickly dip half of the ladyfingers into the cooled espresso and arrange them in a single layer in the bottom of the baking dish. Spread half of the mascarpone mixture evenly over the ladyfingers.
Dip the remaining ladyfingers into the espresso and arrange them on top as a second layer. Spread the remaining mascarpone mixture evenly over the top and finish with a generous dusting of cocoa powder.
Refrigerate for at least 6 hours before serving. Overnight is even better, as it allows the flavors to develop and the tiramisu to set perfectly.
Storage
Cover the tiramisu tightly and store it in the refrigerator for up to 2–3 days.
You can also freeze it in an airtight container for up to 3 months. Thaw overnight in the refrigerator before serving. The texture may change slightly after freezing.