In a mixing bowl, combine all-purpose flour, powdered sugar, unsalted butter, and an egg yolk. Mix until it resembles coarse crumbs. Form into a disc, wrap in plastic wrap, and chill in the refrigerator for 30 minutes.

Preheat your oven to 375°F (190°C). Roll out the chilled crust until about 1/8 inch thick, then transfer it to a tart pan and press into place. Line with parchment paper and fill with weights. Bake for 15-20 minutes or until lightly golden. Remove weights and let cool completely.

In a small saucepan, heat heavy cream over medium heat until it simmers. Pour it over chopped dark chocolate in a bowl. Let sit for 5 minutes, then stir until smooth and glossy.

In a skillet over medium heat, add granulated sugar and cook, stirring until it melts and turns golden. Add toasted hazelnuts and stir until well-coated. Pour onto parchment paper to cool.

Once the tart crust is cooled, pour the smooth ganache into the crust and spread evenly. Chill in the refrigerator for about 2 hours, until set.

Before serving, top with cooled candied hazelnuts in a decorative pattern. Slice and serve chilled.