6 egg yolks
3/4 cup sugar
1 lb. mascarpone cheese, softened
2 cups chilled heavy cream
2 Tbs. rum
1 tsp. vanilla extract
2 Tbs. plus 2 to 3 cups brewed espresso
5 egg whites
40 to 50 ladyfingers
Cocoa powder for dusting
In a mixing bowl set over a pan of simmering water but not touching the water, whisk the egg yolks and sugar until the mixture is pale yellow and thick ribbons fall from the whisk, 4 to 5 minutes. Transfer the mixture to the bowl of an electric mixer. Add the mascarpone cheese and beat until smooth and creamy, 2 to 3 minutes.
In a chilled large mixing bowl, whisk the cream until stiff peaks form. Add the rum, vanilla and the 2 Tbs. espresso and whisk until smooth.
In a clean mixing bowl, whisk the egg whites until stiff peaks form. (When the whisk is lifted straight out of the bowl and inverted, the whites should hold their shape.)
With a rubber spatula, gently fold the mascarpone mixture into the cream until blended and smooth. Add about 1 cup egg whites and fold gently until blended. Add the remaining egg whites and fold gently until the mixture is smooth and blended.
One at a time, submerge the ladyfingers into the 2 to 3 cups espresso. Lay enough ladyfingers on the bottom of a 6-quart glass or ceramic baking or serving dish (about 2 inches deep) to form a single layer. Spread half of the mascarpone mixture over the ladyfingers until evenly covered. Arrange another layer of ladyfingers over the mascarpone cream, then spread the remaining mascarpone cream evenly over the top. Dust the tiramisù with cocoa powder to create a rich, dark topping. Refrigerate for at least 4 hours or up to 1 day before serving.