In a medium bowl, whisk together the egg yolks and whole eggs with the sugar until smooth and pale. In a separate saucepan, mix the cornstarch and salt, then gradually pour in the whole milk and heavy cream. Heat this mixture over medium-low until it's just below boiling, stirring frequently. Remove from heat and add the chopped dark chocolate, stirring until completely melted and the custard is silky.

In a mixing bowl, combine the all-purpose flour, icing sugar, cocoa powder, and salt. Mix until well blended, then add the melted butter and vanilla extract. Stir the mixture until it forms a cohesive dough.

Preheat your oven to 350°F (175°C). Firmly press the tart dough into a flan ring or pie dish, ensuring even layer and well-defined edges. Pour the rich chocolate custard over the tart base and bake for about 35-40 minutes, or until the custard is just set but still slightly jiggly in the center.

Remove the flan from the oven and let it cool at room temperature for about 30 minutes. Once cooled, transfer to the refrigerator and chill for a minimum of 4 hours. Before serving, brush the top with a tablespoon of jam for shine.