In a large mixing bowl, whisk together 200 g of flour, 100 g of granulated sugar, 40 g of cocoa powder, and ¼ teaspoon of salt until well combined. Cut in 120 g of cold unsalted butter until the mixture resembles coarse crumbs. Add 1 egg and 1 egg yolk, mixing until a dough forms. Wrap the dough in plastic wrap and refrigerate for 30 minutes.

While the dough chills, preheat your oven to 180°C (350°F). Prepare your 9-inch tart pan by greasing it lightly.

On a lightly floured surface, roll out the chilled dough until it's about ¼ inch thick and large enough to fit into your tart pan. Transfer the rolled dough into the prepared pan, pressing it into the bottom and up the sides. Prick the bottom with a fork to prevent bubbling and refrigerate for an additional 10 minutes.

Once the crust is chilled, bake it in the preheated oven for 15-20 minutes, or until it looks set and slightly firm to the touch. Allow it to cool completely.

In a saucepan, heat 400 ml of milk over medium heat until just below boiling. In a separate bowl, whisk together 1 egg yolk, 100 g of granulated sugar, and 2 tablespoon of flour until smooth. Gradually mix in the hot milk, whisking constantly to prevent curdling. Return the mixture to the saucepan and cook over low heat until it thickens, about 3-5 minutes.

Remove the filling from heat and add 200 g of dark chocolate, stirring until completely melted and smooth. Allow the filling to cool slightly before pouring it into the cooled crust.

Carefully pour the chocolate filling into the prepared crust, spreading it evenly with a spatula. Tap the tart pan on the counter to remove any air bubbles.

Refrigerate the assembled tart for at least 30 minutes, allowing the filling to set properly.