chocolate crust
6 tablespoons (3 ounces/85g) salted butter, at room temperature
1/4 cup (35g) powdered sugar
1 large egg yolk
1 cup (140g) all-purpose flour
1/3 cup (35g) Dutch-process or natural unsweetened cocoa powder
1/4 teaspoon fleur de sel or other flaky sea salt

filling
8 ounces (230g) bittersweet or semisweet chocolate, chopped
2 large eggs
1 1/4 cups (310ml) whole milk
1/2 teaspoon vanilla extract, or
1 teaspoon dark rum
1 cup (240g) dulce de leche
Flaky sea salt, for sprinkling over the tart
Whipped cream or vanilla ice cream for serving, optional

To make the crust, in the bowl of a stand mixer fitted with the paddle attachment, beat the butter and powdered sugar on low speed just until smooth. Add the yolk, stopping the machine to scrape down the sides of the bowl, until it’s fully incorporated.
In a small bowl, whisk together the flour and cocoa powder. Add them to the butter mixture, mixing just until the dough comes together. Form the dough into a disk, wrap in plastic,and let rest for 30 minutes at room temperature.
Use the heel of your hand to press the dough into a 9-inch (23cm) tart ring with a removable bottom, getting the bottom as flat as possible and pressing the dough up the sides of the pan until it reaches the rim. Sprinkle the salt over the bottom of the dough and press it into the pastry. Put the pan in the freezer for 30 minutes.
Preheat the oven to 400ºF (200ºC). Line the chilled tart crust with aluminum foil and cover with a layer of pie weights or dried beans. Bake the tart shell for 15 minutes, remove the foil and pie weights, and then bake for 5 minutes more, until the tart shell feels set. Remove from the oven and decrease the oven temperature to 300ºF (150º).
While the tart is baking, make the chocolate filling. Melt the chocolate in a clean, dry bowl set over a pan of simmering water. Once melted, remove the bowl from the heat and set a fine-mesh strainer over the top.
Whisk the eggs in a bowl. Heat the milk in a saucepan, then gradually whisk the warm milk into the eggs. Scrape the mixture back into the saucepan and cook over medium heat, stirring constantly with a heatproof spatula, until it’s steamy and thickens slightly, about 3 minutes. (If it separates a bit, remove it from the heat, and whisk it vigorously to bring it back together.) Pour the custard through the strainer into the chocolate. Add the vanilla and stir until smooth.
Spread the dulce de leche over the hot tart shell in an even layer, being careful as you spread to make sure you don’t break the flaky bottom of the tart. (If the dulce de leche is very thick, let it sit in the tart shell for a minute or so, to let the heat soften it, which will make it easier to spread.) Set the filled tart shell
on a baking sheet lined with aluminum foil, then pour the chocolate custard over the dulce de leche, smooth the top, and add a generous sprinkling of flaky sea salt.
Bake the tart for 20 minutes, and then turn the heat off and leave the tart in the oven with the door closed to glide to a finish, 25 minutes more. Remove from the oven and let cool before serving. Serve the tart with softly whipped cream, vanilla ice cream, or just as is.