Combine the all-purpose flour, unsweetened cocoa powder, icing sugar, almond flour, and cold unsalted butter until sandy.

Add the medium egg and salt, then mix only until a cohesive dough forms.

Roll the dough to 3 mm thickness between two sheets of parchment paper.

Chill the rolled dough in the refrigerator for at least 60 min.

Line the 19 cm tart ring with the chilled dough and trim the excess.

Freeze the lined tart shell for 30 min, then bake it at 160°C / 320°F for about 25 min.

Cool the baked tart shell completely before filling.

Finely chop the dark chocolate and place it in a mixing bowl.

Heat 80 ml of heavy cream until boiling, then pour it over the dark chocolate.

Whisk the dark chocolate mixture smooth, then fold in the roasted hazelnuts.

Pour the hazelnut ganache into the cooled tart shell and chill until firm.

Soak the gelatin sheets in cold water until soft, then squeeze out excess water.

Heat the whole milk, remaining 90 ml heavy cream, and honey until hot but not boiling.

Stir the softened gelatin sheets into the hot milk mixture until fully dissolved.

Melt the white chocolate, then add the hot milk mixture in three additions.

Add room-temperature mascarpone and blend for about 1 min with the immersion blender kept at the bottom.

Pour the mascarpone mixture into the silicone mold and freeze for about 300 min.

Unmold the frozen mascarpone insert and spray it with white cocoa butter spray, if desired.

Place the mascarpone insert over the firm ganache and decorate with red currants.

Finish the tart with chocolate bells and refrigerate until serving.