In a large bowl, sift the flour, icing sugar and salt. Add the butter. Using your fingertips, or a handheld dough blender, incorporate the butter into the flour mixture until it becomes crumbly.

Make a well in the center and add the egg yolks. Incorporate with your fingertips, or with the handheld blender, until a dough forms (if necessary, add 1 or 2 Tbs of iced water).

On a floured work surface, pat the dough into a 1-inch-thick disk. Wrap the disk in plastic wrap and refrigerate for at least one hour. On a lightly floured work surface, roll out the dough into a 1/8 inch thick round about 2 inches in diameter wider than the bottom of the pan. Ease the dough round into a tart pan with a removable bottom. Rolling the rolling pin over the pan will easily trim any overhanging dough. Prick the bottom of the dough with a fork, cover with plastic wrap and freeze for 30 minutes.

Preheat the oven the 375ºF. Line the frozen pie with parchment paper and fill with pie weights or dry beans. Bake for about 15 minutes. Remove the weights and the parchment paper and bake for about 8 minutes longer, until the edges become golden.

Remove the tart from the oven and let cool for a minimum of 2 hours before filling.

While the tart is baking, make the chocolate filling. Melt the chocolate in a bowl set over a pan of simmering water. Once melted, add the butter and mix with a spatula or with a wooden spoon. Remove the bowl from the heat.

In a bowl, whisk the eggs. Heat the milk in a saucepan over medium heat and gradually incorporate the eggs. Cook, stirring constantly with a wooden spoon, until it thickens slightly, about 3 minutes. Place a fine mesh strainer over the bowl containing the chocolate and pour the milk and egg mixture through it into the chocolate. Add the vanilla and stir until smooth.

Spread the dulce de leche over the hot tart shell in an even layer (the heat of the tart shell will soften the dulce de leche a bit, making it easier to spread). Pour the chocolate custard over the dulce de leche and smooth the top. Bake the tart for 20 minutes, then turn the heat off and leave the tart in the oven with the door closed for another 35 minutes. Remove from the oven and let cool before serving.

Serve the tart with whipped cream or with creme fraiche, or just as is.