Combine the all-purpose flour, unsweetened cocoa powder, icing sugar, hazelnut flour, and salt in a mixing bowl. Rub in the cold unsalted butter until the mixture resembles fine crumbs.

Add the egg and mix just until a dough forms. Flatten the dough into a disk.

Wrap the dough and chill it in the refrigerator for 60 min. Roll it to 3 mm thickness between two sheets of parchment paper.

Line the 20 cm springform pan with the cocoa pastry. Freeze the lined shell while preparing the filling.

Heat the whole milk with the split vanilla bean and its seeds in a saucepan until nearly boiling. Remove the vanilla bean.

Whisk the egg yolk with the granulated sugar until smooth. Whisk in the starch until no lumps remain.

Gradually whisk the hot vanilla milk into the egg yolk mixture. Return the mixture to the saucepan.

Cook over medium heat while whisking until the custard becomes thick. Remove the saucepan from the heat.

Stir 100 g of the dark chocolate into the hot custard until melted. Stir in the butter until fully incorporated.

Whip 100 ml of the heavy cream to soft peaks. Fold it into the lukewarm chocolate custard.

Transfer the custard to a baking dish and cover it with plastic wrap touching the surface. Chill until cool.

Pour the chilled chocolate custard into the frozen pastry shell. Level the surface with a rubber spatula.

Bake at 170°C / 340°F for 55 min. Leave the center slightly wobbly because it will set as it cools.

Heat the remaining 40 ml heavy cream until hot. Pour it over the remaining 40 g dark chocolate and stir into a smooth ganache.

Spread the ganache over the cooled flan. Chill the finished flan for 360 min before serving.

Melt extra dark chocolate if desired and pipe it onto an acetate strip. Chill the decorations, then place them on the flan.