In a mixing bowl, combine the flour, powdered sugar, and salt.

Add the cold butter and mix with your fingertips until the texture becomes crumbly.

Stir in the egg and mix until the dough comes together. Form a ball, wrap it in plastic wrap, and refrigerate for 30 minutes.

Preheat the oven to 350°F (180°C).

Roll out the dough on a lightly floured surface and press it into a 9-inch tart pan.

Prick the bottom of the crust with a fork to prevent puffing. Line with parchment paper and fill with pie weights or dried beans.

Bake for 15 minutes, then remove the weights and parchment paper and bake for an additional 10 minutes until lightly golden. Let it cool completely.

Heat the heavy cream in a saucepan until it just begins to simmer.

Remove from heat and pour over the chopped dark chocolate. Let it sit for 1 minute, then stir until smooth.

Add the butter and mix until fully incorporated for a glossy finish.

Heat a dry skillet over medium heat and toast the hazelnuts for 3-5 minutes, stirring frequently until fragrant.

Let them cool, then roughly chop.

Pour the chocolate ganache into the cooled tart crust, spreading it evenly.

Sprinkle the toasted hazelnuts over the top, gently pressing them into the ganache.

Let the tart sit at room temperature for 1 hour or refrigerate for a firmer texture.