Preheat the oven to 350°F (175°C).
Combine flour, cocoa, and powdered sugar, then cut in the cold butter.
Mix in the egg yolk and enough cold water to bring the dough together.
Press the dough into a 9-inch tart pan, prick the bottom, and bake for 18–20 minutes.
Place the chopped chocolate in a heatproof bowl.
Heat the cream until steaming, pour it over the chocolate, and let stand for 2 minutes.
Stir until smooth, then add butter, vanilla, and half of the hazelnuts.
Pour the ganache into the cooled crust, top with remaining hazelnuts, and refrigerate for at least 2 hours before slicing.