Begin with the crust. In a mixing bowl, combine the flour and powdered sugar. Add the cold butter cubes and use your fingers or a pastry cutter to mix until the mixture resembles coarse crumbs. Beat the egg yolk lightly and mix it in, adding cold water a little at a time until the dough comes together.
Press the dough into your tart pan evenly, making sure to cover the bottom and sides. Use the back of a spoon to smooth it out. Chill the crust in the refrigerator for at least 15 minutes. This helps it stay firm during baking and ensures a crisp texture.
Preheat the oven to 350 degrees Fahrenheit. Line the crust with parchment paper or foil and fill it with pie weights or dry beans. Bake for 15 minutes, then remove the weights and bake for another 5-10 minutes until lightly golden. Let the crust cool completely on a cooling rack.
While the crust cools, prepare the chocolate filling. In a saucepan, heat the heavy cream over medium heat until it just begins to simmer. Remove from heat and pour over the chopped chocolate in a heatproof bowl. Let it sit for 2-3 minutes, then stir until smooth and glossy. Add butter, vanilla, and a pinch of salt, stirring until fully combined.
Pour the chocolate filling into the cooled tart shell. Smooth the top with a spatula and sprinkle the toasted hazelnuts evenly over the surface. You can press them gently into the chocolate to make sure they stick.
Chill the tart in the refrigerator for at least 2 hours or until the filling is set. When ready to serve, dust lightly with powdered sugar or cocoa powder if desired. Slice carefully using a sharp knife, wiping the blade between cuts for clean slices. Enjoy the rich chocolate and nutty goodness with your favorite cup of coffee or tea.