In a mixing bowl, combine all-purpose flour, icing sugar, cocoa powder, and salt, then add melted unsalted butter. Mix these ingredients until the mixture resembles coarse crumbs. Press the mixture firmly into the base of a tart dish, ensuring an even thickness. Chill in the refrigerator for at least 30 minutes to set while you prepare the chocolate custard.

In a large bowl, whisk together egg yolks, whole eggs, and white sugar until the mixture turns pale and creamy. Gradually blend in cornstarch and salt, continuing to whisk for consistency. In a saucepan, heat whole milk and heavy cream over medium heat until steaming but not boiling. Slowly pour the hot milk mixture into the eggs, whisking constantly to avoid curdling. Finally, melt dark chocolate and stir it into the custard until smooth and well combined.

Preheat your oven to 350°F (175°C). Once the tart base has chilled, pour the luxurious chocolate custard over it, spreading it evenly. Place the tart in the oven and bake for 30-35 minutes, watching carefully. The custard should be just set but still slightly jiggle in the center—a perfect sign of a silky French chocolate flan.

After baking, remove the flan from the oven and allow it to cool completely at room temperature. Once cooled, brush the top with your choice of apricot or raspberry jam for a beautiful glossy finish. Chill in the refrigerator for a few hours before serving to enhance the flavors and ensure the dessert holds its shape upon slicing.