In a mixing bowl, combine all-purpose flour, icing sugar, cocoa powder, and salt. Mix in melted butter and vanilla extract until a dough forms. Press into a tart pan and chill for at least 30 minutes.

In a separate bowl, whisk together egg yolks, whole eggs, sugar, cornstarch, and salt until pale and well combined. Heat milk and cream in a saucepan until just boiling, then gradually pour over the egg mixture while whisking continuously.

Stir in chopped dark chocolate into the warm custard until fully melted and smooth.

Preheat oven to 350°F (175°C). Pour chocolate custard into the prepared tart crust. Bake for 45-50 minutes, until edges are set with a slight jiggle in the center.

Allow the flan to cool at room temperature for about 30 minutes. Brush the top with jam and refrigerate for at least 2 hours to set.