In a mixing bowl, combine all-purpose flour, icing sugar, cocoa powder, and a pinch of salt. Melt the butter and mix it with the vanilla extract before combining into the dry ingredients. Use your hands or a spatula to form a dough. Press the dough evenly into a tart pan, covering the base and sides, and chill in the refrigerator for about 30 minutes to firm up.

In a large bowl, whisk together the egg yolks, whole eggs, and white sugar until the mixture becomes pale and creamy. Meanwhile, in a saucepan, heat the whole milk and heavy cream over medium heat until it is warm. Gradually whisk the warm milk mixture into the egg mixture to avoid cooking the eggs and then stir in the cornstarch and salt until smooth.

Melt the dark chocolate in a microwave or using a double boiler, stirring until smooth. Allow it to cool slightly before folding it into the custard mixture gently.

Preheat your oven to 325°F (160°C). Pour the chocolate custard into the chilled tart shell, filling it almost to the top. Bake for about 30-35 minutes until the custard is just set but still slightly jiggly in the center.

Remove the flan from the oven and allow it to cool at room temperature for about an hour. Cover and refrigerate for a minimum of 2 hours or overnight if possible.