1 teaspoon vanilla extract chocolate, chilled, for garnish
In a large bowl, whisk together yolks and mascarpone until smooth. Add the granulated sugar and whisk until dissolved. Set aside.
Place biscotti in the bowl of a food processor; pulse to form coarse crumbs. Set aside.
In the bowl of an electric mixer fitted with the whisk attachment, add egg whites; beat until soft peaks form.
Fold egg whites into mascarpone mixture; set aside.
Pour the cooled espresso into a shallow dish and quickly dip the ladyfingers individually into the espresso, and then place in the bottom of a 13-by-9-inch glass baking dish. Evenly spread half the mascarpone mixture over ladyfingers. Sprinkle with half of the biscotti crumbs. Repeat layers.
In a large bowl, whisk together heavy cream, confectioners' sugar, and vanilla until soft peaks form. Spread whipped cream evenly over tiramisu.
Using a fine grater, garnish the top of the dish with grated chocolate and a few of the biscotti crumbs. Chill at least 2 hours and preferably overnight before serving.