Rub the flour, cocoa powder, powdered sugar, and salt together with the cold diced butter using your hands. This is called the 'sanding' method. It's best to use a food processor or stand mixer to keep the butter cold.

Transfer the mixture to a bowl, add the egg, and quickly mix to form a dough. Shape into a disk, wrap in plastic wrap, and refrigerate for at least 2 hours.

Roll out the dough with a rolling pin and line a 11-inch tart pan (28 cm). Prick the bottom with a fork and cover with parchment paper. Fill with pie weights, uncooked rice, or dried beans.

Bake in a preheated oven at 340°F (170°C) for 15 minutes. Remove the parchment and weights, then bake for another 5 minutes.

Use any leftover dough to make small cookies for decorating the tart. You can use any cookie cutter shape you like. Bake them for 8–10 minutes at 340°F (170°C).

Heat the cream with the sugar, stirring until the sugar dissolves. Remove from heat and add the well-drained gelatin, stirring until fully dissolved. Stir in the mint syrup and let the mixture cool until almost at room temperature.

Chill in the refrigerator for at least 5–6 hours, or overnight. Decorate with the cookies and dark chocolate as desired.