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

Preheat your oven to 350°F (175°C). Ensure your oven has reached this temperature before proceeding.

Line the chilled tart crust with parchment paper and fill it with baking weights. Bake for 15-20 minutes until lightly golden. Remove the weights and parchment, then bake for an additional 5 minutes.

In a medium saucepan over medium heat, whisk together the egg yolks, whole eggs, sugar, cornstarch, and salt until well combined. Gradually whisk in the whole milk and heavy cream. Heat until thickened, about 6-8 minutes. Remove from heat and stir in dark chocolate until melted.

Carefully pour the chocolate custard into the cooled tart crust and bake for 30-35 minutes until just set. The center should jiggle slightly while the edges are firm.

Once baked, cool at room temperature for 30 minutes, then refrigerate for at least 4 hours. Brush with jam before serving.