Add mascarpone to the egg yolk mixture and mix until well combined.
In a separate bowl, whip the cream to stiff peaks using a stand or hand-held electric mixer.
Using a rubber spatula, gently fold the whipped cream into the zabaglione mixture. Set aside.
In a shallow dish, mix together the cooled espresso or strong coffee with the Kahlua. Quickly dip the ladyfingers into the coffee, 1 second each side, being careful not to soak them.
Arrange the ladyfingers in the bottom of a 9 x 13 inch baking dish (or similar size). You can cut the ladyfingers to fit.
Spoon half of the tiramisu cream filling over the ladyfingers. Repeat with a second layer of ladyfingers. Add another layer of tiramisu cream.
Cover and refrigerate for at least 6 hours or overnight.
Right before serving, dust with cocoa powder.