In a medium bowl, whisk together the egg yolks and whole eggs until smooth. Gradually add the white sugar and cornstarch, mixing until blended. Heat whole milk and heavy cream in a saucepan over medium heat until simmering. Remove from heat and slowly pour over the egg mixture, stirring constantly. Fold in melted dark chocolate until combined.

In a large mixing bowl, combine all-purpose flour, icing sugar, cocoa powder, and salt. Mix well. Add melted butter and vanilla extract, stirring until a dough forms. Press evenly into a 9-inch tart pan. Chill for 15-20 minutes while preheating the oven.

Preheat oven to 350°F (175°C). Pour the chocolate custard into the chilled tart crust, filling just below the edge. Place the tart pan in a larger baking dish and fill with hot water halfway up the sides. Bake for 40-45 minutes, until the custard is set but slightly wobbly.

Remove from oven and let cool at room temperature for 30 minutes. Cover and refrigerate for at least 4 hours or overnight for best flavor. Adjust jam topping before serving.

When ready to serve, brush the flan with fruit jam for shine. Slice into wedges and serve chilled, optionally garnished with fresh berries or whipped cream.