In a mixing bowl, whisk together the flour, powdered sugar, and salt. Add cold, cubed butter and rub it into the flour mixture using your fingertips until it resembles coarse crumbs. Mix in the egg yolk and cold water until the dough comes together. Shape the dough into a disk, wrap in plastic wrap, and chill for 30 minutes. Roll out the chilled dough and cut into circles to fit tartlet molds. Press the dough into the molds and prick the bottom with a fork. Preheat oven to 180°C (350°F) and blind bake for 15 minutes with pie weights, then another 5 minutes without them until golden. Allow tart shells to cool completely before filling.

Heat heavy cream in a saucepan over medium heat until it just begins to simmer. Pour over the chopped dark chocolate and let sit for 1 minute. Stir gently until smooth and fully melted. Mix in ground hazelnuts, butter, honey, and vanilla extract until well combined. Pour the ganache into the cooled tart shells and refrigerate for 1 hour until set.

Heat heavy cream and pour over chopped dark chocolate. Stir until smooth and glossy. Add hazelnut spread for extra flavor (optional). Spread a thin layer of glaze over the chilled tarts and top with chopped hazelnuts. Let set in the refrigerator for another 30 minutes before serving.