In a large mixing bowl, combine all-purpose flour, icing sugar, cocoa powder, and salt. Pour in melted butter and vanilla extract, stirring until a soft dough forms. Press the mixture evenly into a tart pan, then chill in the refrigerator for 30 minutes.

Preheat your oven to 350°F (175°C). Bake the chilled tart crust for 15-20 minutes, or until set. Remove from oven and let cool completely.

In a medium saucepan, whisk together egg yolks, whole eggs, white sugar, cornstarch, and salt. Gradually add whole milk and heavy cream while whisking continuously until thickened, about 8-10 minutes.

Remove the saucepan from heat and stir in chopped dark chocolate until melted and smooth.

Pour the chocolate custard into the cooled tart shell. Bake in a water bath for 30-35 minutes until slightly jiggly in the center.

Let the flan cool to room temperature, brush the top with jam, cover, and refrigerate for at least 2 hours.