Pour milk into a medium heavy-bottomed saucepan. Split vanilla bean and, with the tip of a sharp knife, scrape all the black paste inside it into the milk. Add the pod. Heat over medium heat until very hot and bubbles form around the edges, but do not allow it to boil, 3 to 5 minutes. Remove from heat and let stand for 5 minutes. Discard the pod.Advertisement
Whisk egg yolks and 2/3 cup sugar in a medium bowl until pale yellow. Whisk in flour. Dribble in a little of the hot milk, whisking as you go, to prevent the eggs from cooking. Continue to slowly whisk in milk until you have added about half.
Slowly whisk the egg mixture back into the remaining milk in the pan and return to medium heat. Cook, whisking constantly, until the mixture has thickened and is just about to boil. Immediately turn heat to low and continue to cook, whisking, until very thick, 3 to 4 minutes.
Place a fine-mesh sieve over a heatproof bowl. Strain the pastry cream through the sieve, scraping the underside to make sure you get it all. Let cool 5 minutes. Press plastic wrap directly on the surface of the cream and refrigerate until cold, at least 1 1/2 hours and up to 1 day.
Whip heavy cream, mascarpone and the remaining 3 tablespoons sugar in a large bowl until stiff. Reserve 2 cups of the whipped cream, then gently fold the rest into the chilled pastry cream.
Arrange half the Pandoro slices snugly in the bottom of a 9-by-13-inch baking dish. Combine espresso and Grand Marnier (or juice). Brush half the espresso mixture over the cake and sprinkle with 2 tablespoons chocolate. Spread half of the pastry cream mixture over the cake. Arrange the remaining Pandoro slices over the pastry cream, brush with the remaining espresso mixture and sprinkle with the remaining 2 tablespoons chocolate. Spread the remaining pastry cream evenly over the top, then spread the remaining whipped cream over that. Cover tightly with plastic wrap. Refrigerate for at least 8 hours and preferably overnight.
Right before serving, dust the top of the tiramisù with cocoa. Cut into 18 pieces.
Tip: Pandoro cake is a tall, star-shaped Christmas bread with origins in Verona, Italy. It has an airy texture and sweet flavor. Traditionally, Italians give these cakes to symbolize luck and prosperity in the new year. Find them at Italian and gourmet shops or online at supermarketitaly.com.
To make ahead: Refrigerate for up to 3 days.
4 1/2 fat, 1 other carbohydrate