Oops, I Dropped the Lemon Tart

Ingredients:
3 1/4 cups whole milk
1 cup heavy cream
3/4 cup sugar
10 stems lemongrass (bottom third only, tough outer layers removed), lightly smashed
Finely grated zest of 2 lemons
3 1/2 cups all-purpose flour
"1 1/2 cups confectioners sugar", 1 teaspoon ground spices, such as star anise, cinnamon, juniper, cardamom, or black pepper (optional)
3 sticks (3/4 pound) unsalted butter, cut into tablespoons and chilled
4 large egg yolks, chilled
5 large egg yolks, at room temperature
1/2 cup sugar
1/4 cup plus 3 tablespoons freshly squeezed lemon juice
1/4 cup plus 2 tablespoons limoncello, preferably from Amalfi
Equipment: Instant-read thermometer
fine-mesh sieve
ice cream maker
2 (11-inch) tart pans
pie weights or dried beans

Instructions:
Fill a sink or large stainless steel bowl with ice and arrange a second slightly smaller bowl on top of the ice.
In a heavy medium saucepan over moderate heat, combine the milk, cream, sugar, lemongrass, and lemon zest. Cook, stirring often with a wooden spoon, until an instant-read thermometer registers 160°F. Pour through a fine-mesh sieve into the stainless-steel bowl set over ice. Let the mixture cool, whisking occasionally, until chilled.
Process custard in an ice cream maker then transfer to an airtight container and freeze until hard, at least 4 hours and up to 1 week.
In a large bowl, stir together the flour and confectioners' sugar. Add the spices, if desired, and stir to incorporate. Add the butter and, using your fingertips or 2 knives, work it into the flour and confectioners' sugar until the mixture resembles coarse meal. Add the egg yolks and use your hands to mix the dough until it comes together. Divide the dough in half then knead each piece lightly and form into disks. Wrap the dough in plastic wrap and chill it in the refrigerator for 10 minutes.
Arrange a rack in the lower third of the oven and preheat to 350°F.
On a lightly floured surface with a lightly floured rolling pin, roll 1 piece of dough into a 14-inch round. Fold the dough in half, fold it in half again, and then unfold it into an 11-inch tart pan. Using your fingertips, gently fit the dough into the bottom and sides of the tart pan, then use a small sharp knife to trim any excess dough. Using a fork, prick the bottom of the tart shell all over then freeze for 10 minutes. Repeat with the second piece of dough.
Line the tart shells with foil, fitting the foil into the bottom edges, and then fill each with about 1 cup pie weights or dried beans. Bake the shells in the lower third of the oven for about 30 minutes, until golden. Let cool completely then remove the foil and weights.
In a large saucepan, bring 1 inch of water to a simmer.
In a large copper or stainless steel bowl, combine the egg yolks and sugar and use a whisk to beat until thickened and light yellow in color. Place the bowl over the pan of simmering water and turn off the heat. Gradually beat in the lemon juice, followed by the limoncello. Start beating more vigorously and continue beating until the zabaione is fluffy.
Layer the ice cream and zabaione into the tart shells. You can serve the dessert like Chef Bottura does, upside down and somewhat smashed, or you can serve right side up.