Classic French Prune Tart with Creamy Custard

In a medium mixing bowl, whisk together the sifted flour, powdered sugar, and salt for the pâte sucrée. Add the cold cubed butter and work it in with your fingertips or a pastry blender until the mixture looks like coarse meal with pea-sized butter pieces still visible. Make a well in the center and pour in the lightly beaten egg. Mix with a fork or your hands just until a rough dough forms; do not overwork.

Shape the dough into a flat disc about 2 cm thick, wrap tightly in plastic wrap, and refrigerate for at least 1 hour 30 minutes, or up to 48 hours. The dough should feel cold and firm but pliable when you are ready to roll.

Meanwhile, prepare the prune marinade. In a medium heavy-bottomed saucepan, combine the water, granulated sugar, Earl Grey tea bags, vanilla extract, cinnamon stick, and lemon zest strips. Bring to a gentle boil over medium heat, stirring to dissolve the sugar. Reduce the heat and simmer for 5 minutes so the tea and aromatics infuse the syrup.

Remove the saucepan from the heat. Stir in the Armagnac or rum if using. Add the pitted prunes and push them down so they are mostly submerged in the hot syrup. Cover and set aside to cool to room temperature.

Transfer the covered saucepan to the refrigerator and marinate the prunes for at least 8 hours, preferably overnight. The prunes will absorb the syrup and become plump, silky, and deeply flavored.

On a lightly floured surface, roll the chilled dough into a round about 32 cm in diameter and 3 mm thick. Transfer to the 24 cm tart pan and gently press into the base and up the sides. Trim the overhang flush with the rim. Prick the base all over with a fork to prevent puffing, then freeze the lined shell for 15 minutes to firm up the butter.

Preheat the oven to 180°C (350°F). Line the chilled tart shell with a sheet of parchment paper and fill with pie weights or dried beans up to the edges. Blind bake for 20 minutes, then carefully lift out the parchment and weights.

Return the shell to the oven and bake for another 8 to 10 minutes at 180°C (350°F), until the base is pale golden and feels dry to the touch. Transfer to a cooling rack and reduce the oven temperature to 160°C (320°F).

Drain the marinated prunes in a fine-mesh strainer set over a bowl, reserving the syrup for another use if desired. Pat the prunes gently with paper towels so they are moist but not dripping; excess liquid will make the finished custard weep.

Arrange the drained prunes in a single snug layer over the warm tart shell, setting them on their sides or cut side down so the curved backs face up. Pack them in tightly so they support one another; any small gaps will fill with custard.

Make the custard: in a medium saucepan, combine the whole milk, heavy cream, sugar, split vanilla bean with its scraped seeds, and salt. Warm over medium-low heat, whisking occasionally, until the sugar dissolves and small bubbles appear around the edges. Do not let it boil. Remove from the heat, cover, and steep for 10 minutes to infuse the vanilla.

In a large mixing bowl, whisk the three eggs just until the yolks and whites are combined; do not whip in air. Gradually pour the warm cream mixture into the eggs in a thin, steady stream while whisking constantly to temper them. Strain the custard through a fine-mesh strainer into a large measuring cup or pitcher to remove the vanilla pod and any bits of cooked egg. Skim off any surface foam with a spoon.

Carefully pour the strained custard over the arranged prunes, filling the shell almost to the rim. Gently tap the pan on the counter two or three times to release any trapped air bubbles and let the custard settle evenly around the prunes.

Bake the tart on the middle rack at 160°C (320°F) for 35 to 40 minutes, until the custard is just barely set 3 cm from the edge but still has a soft wobble in the center. The internal temperature of the custard should reach 71°C (160°F) in the center for safe consumption of the egg-based custard. The surface should be pale gold with no brown spots. If the rim of the crust browns too quickly, tent it loosely with foil.

Cool the tart in the pan on a cooling rack for at least 1 hour 30 minutes. The custard will finish setting as it cools; do not cut into it while warm or it will slump. Once fully cooled, the tart can be served at cool room temperature or after a brief 30-minute chill in the refrigerator.

Release the tart from the pan by loosening the rim, transfer to a serving plate, and slice with a thin sharp knife wiped clean between cuts. Serve plain, with a small dollop of crème fraîche, or dusted lightly with powdered sugar.