In a mixing bowl, combine 1 cup all-purpose flour, 1/4 cup icing sugar, 1/4 cup cocoa powder, and a pinch of salt. Pour in 1/2 cup melted butter and 1 teaspoon vanilla extract; mix until crumbly. Press this mixture firmly into the bottom and up the sides of a tart pan. Chill in the refrigerator for at least 30 minutes to set before baking.

In a large bowl, whisk together 4 egg yolks, 2 whole eggs, 1/2 cup white sugar, 2 tablespoons cornstarch, and 1/4 teaspoon salt until smooth and creamy. Gradually whisk in 2 cups of whole milk and 1 cup heavy cream until well blended, ensuring there are no lumps. Set this custard mixture aside while you prepare the chocolate.

Place 6 ounces of chopped dark chocolate in a heatproof bowl. Gently melt the chocolate over a double boiler, stirring frequently until smooth and glossy, about 5 minutes. Once melted, remove from heat and slowly incorporate the warm custard mixture into the chocolate, whisking continuously to maintain a silky texture.

Preheat your oven to 350°F (175°C). Pour the combined chocolate custard mixture into the prepared tart crust. Carefully place the tart pan into the oven and bake for 45–50 minutes, or until the center of the flan is slightly jiggly but the edges are set. Remove from the oven and allow to cool at room temperature.

After cooling, brush the top of the French chocolate flan with a thin layer of jam for a beautiful glossy finish. Transfer it to the refrigerator to chill for at least 2 hours, allowing the flavors to meld. Once chilled, slice and serve this decadent chocolate dessert with fresh berries or a dusting of cocoa powder, if desired.