In a mixing bowl, whisk together flour, granulated sugar, cocoa powder, and a pinch of salt. Cut in cold butter until the mixture resembles coarse crumbs. Add one whole egg and one egg yolk, mixing until the dough forms. Wrap the dough 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 to about ¼-inch thickness. Transfer the rolled dough into a 9-inch tart pan, pressing it into the edges and trimming any excess. Use a fork to prick the bottom of the crust.

Place the tart shell in the preheated oven and bake for 15–20 minutes or until set. Remove it from the oven to cool completely.

In a saucepan, heat milk over medium heat until just below boiling. In a separate bowl, whisk together egg yolks, sugar, and flour until smooth. Gradually pour the hot milk into the egg mixture while whisking, then return to the saucepan.

Cook the combined mixture over low heat, stirring constantly until it thickens, about 5–7 minutes. Remove from heat and stir in chopped dark chocolate until completely melted and smooth.

Once the tart crust has cooled, pour the chocolate filling into the shell and chill in the refrigerator for at least 30 minutes before serving.

Garnish with chocolate shavings or nuts if desired, slice and serve chilled.