In a large mixing bowl, whisk together flour, granulated sugar, cocoa powder, and salt until well combined.

Cut in cold butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs.

Add whole egg and egg yolk to the dry mixture. Stir until a dough begins to form, pulling it together with your hands if necessary.

Wrap the dough tightly in plastic wrap and refrigerate for at least 30 minutes.

Preheat your oven to 180°C (350°F). Roll out the chilled dough on a lightly floured surface.

Transfer the rolled dough into the tart pan, pressing it into the edges and trimming any excess.

Prick the bottom of the tart shell with a fork to prevent bubbling during baking.

Bake the tart shell in the preheated oven for 15 minutes, then let it cool.

Heat the milk in a saucepan over medium heat, stirring occasionally until just below boiling.

In a separate bowl, whisk together egg yolks, granulated sugar, and flour until smooth.

Slowly pour the hot milk into the egg mixture, whisking constantly. Return to the saucepan and cook until thickened.

Remove from heat and stir in chopped dark chocolate until melted and smooth.

Pour the chocolate filling into the cooled tart shell, smoothing the top with a spatula.

Chill the tart in the refrigerator for at least 30 minutes, then slice and serve with whipped cream or berries.