Prepare the pastry crust by whisking together the flour, granulated sugar, cocoa powder, and salt in a mixing bowl. Cut in the cold, unsalted butter until the mixture resembles coarse crumbs. Add in the egg and yolk, mixing gently until a soft dough forms. Shape into a disc, wrap in plastic wrap, and refrigerate for 30 minutes.

Preheat your oven to 180°C (350°F) while the dough chills. Roll out the chilled dough to about 1/8 inch thick. Fit into a 9-inch tart pan, pressing it into the edges. Prick the bottom with a fork. Refrigerate for an additional 10 minutes before baking.

Bake the tart shell for 15-20 minutes or until firm to the touch. Allow it to cool completely on a wire rack.

Heat the milk in a saucepan over medium heat until just below boiling. Whisk together the egg yolks, sugar, and flour in a separate bowl until smooth. Gradually add hot milk to the egg mixture while whisking constantly.

Return the mixture to the saucepan and stir continuously over medium heat until it thickens and coats the back of a spoon, about 5-7 minutes. Remove from heat and stir in the chopped dark chocolate until smooth.

Pour the smooth filling into the cooled tart shell and spread evenly with a spatula. Tap gently on the counter to remove air bubbles.

Chill the filled tart in the refrigerator for at least 30 minutes before serving. Garnish with chocolate shavings or nuts if desired.