Separate the egg whites from the egg yolks. Whisk the egg yolks with the powdered sugar in a bowl until the mixture whitens. Add the mascarpone and mix well until you get a smooth and thick cream. Set aside.
Beat the egg whites until stiff peaks form using an electric mixer. Gently fold them into the previous mixture using a spatula. Mix gently until you get a light and homogeneous cream.
Pour the strong cold coffee into a shallow plate. Cover a deep dish with parchment paper. Dip half of the ladyfinger biscuits in the cold coffee, then place them flat and side by side in the bottom of the dish. Cover the ladyfinger biscuits with half of the mascarpone mixture.
Dip the remaining ladyfinger biscuits in the cold coffee again, then place them, still flat and side by side, on top. Cover with the remaining mascarpone mixture and smooth the surface with a spatula. Cover the dish with plastic wrap and refrigerate the tiramisu for at least 12 hours before serving.
When serving, remove the tiramisu from the refrigerator, remove the plastic wrap, and sprinkle the surface with cocoa powder. Enjoy this chilled chocolate tiramisu.