In a mixing bowl, combine 3 egg yolks and 2 whole eggs, whisking until blended. Gradually mix in 1 cup of sugar, ⅓ cup of cornstarch, and ½ teaspoon of salt, ensuring there are no lumps. Slowly pour in 2 ½ cups of whole milk and 1 ¾ cups of heavy cream while whisking continuously. Melt 7 oz of dark chocolate in a microwave or double boiler until smooth, then stir it into the mixture until well combined.

Preheat your oven to 325°F (160°C). Carefully pour the prepared chocolate custard into a baking dish or flan mold, making sure it's evenly distributed. Place the dish in the oven and bake for approximately 45-55 minutes. The custard should be slightly jiggly in the center but set around the edges. Remove it from the oven and let it cool for a bit before transferring it to the refrigerator.

While the custard is cooling, start making the tart crust. In a bowl, whisk together 1 cup of all-purpose flour, ⅓ cup of icing sugar, ¼ cup of cocoa powder, and a pinch of salt. Mix in 1 stick of melted butter and ½ teaspoon of vanilla extract until a dough forms. Press this dough into a tart pan, ensuring an even layer, and chill it in the refrigerator for about 30 minutes to firm up.

After chilling, preheat your oven to 350°F (175°C). Remove the tart crust from the refrigerator and bake it for 15-20 minutes, until it's firm to the touch and slightly crisp. Keep an eye on it to prevent over-baking. Once done, take it out of the oven and allow the crust to cool completely on a wire rack.

Once both the chocolate custard and tart crust have cooled, carefully pour the chilled custard into the cooled tart shell. Use a spatula to smooth the top as needed. Cover the assembled flan with plastic wrap or foil and refrigerate it for several hours or ideally overnight to allow the flavors to meld beautifully.

When ready to serve, gently remove the flan from the tart pan. Brush the top with a tablespoon of jam to give it a glossy finish. Slice into wedges and serve chilled, perhaps with a dollop of whipped cream or fresh berries for an extra indulgent touch.