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

Cut in the cold butter until the mixture resembles coarse crumbs.

Add the egg and egg yolk, mixing until a cohesive dough forms, then refrigerate for at least 30 minutes.

Preheat your oven to 180°C (350°F), then roll out the chilled dough to roughly ⅛-inch thick.

Transfer the rolled-out dough into the prepared tart pan, pressing it into the edges and trimming excess.

Bake the tart shell for 15 minutes or until slightly firm, then allow it to cool on a wire rack.

In a saucepan, heat the milk over medium heat until steaming.

Whisk together the egg yolks, granulated sugar, and flour in a separate bowl until smooth.

Gradually pour the hot milk into the egg yolk mixture, whisking constantly to avoid curdling.

Return the mixture to the saucepan and cook over medium heat, stirring continuously until it thickens.

Remove from heat, stir in the dark chocolate until completely melted.

Pour the filling into the cooled tart shell and let cool before refrigerating for at least 30 minutes.

Serve chilled, optionally garnished with chocolate shavings or nuts.