For the crust, pulse the flour, salt and sugar in a food processor until combined. Add the butter and process until the mixture resembles coarse sand. Add the yolk and vanilla extract and mix until the dough forms. Pat the dough into a 1-inch-thick disk, cover tightly with plastic wrap and chill in the refrigerator for 30 minutes.

Remove the dough from the refrigerator and let it warm slightly. On a lightly floured work surface roll out the dough 1/8-inch thick and large enough to fit a 9-inch diameter tart pan with a removable bottom. Carefully transfer the dough to the pan and trim the excess. (Extra dough can be stored in the refrigerator up to 3 days or frozen up to 1 month.) Prick the bottom and sides of the crust with a fork. Refrigerate for 30 minutes.

Preheat the oven to 375 degrees F.

Line the crust with aluminum foil and fill with dried beans, rice or pie weights. Bake for 25 minutes until the tart starts to turn light brown. Remove the foil and weights and bake for an additional 10-15 minutes until golden brown. Let the tart shell cool completely before filling.

Place the hazelnuts in a single layer on a sheet pan. Toast in the oven for 8-10 minutes until browned and fragrant.

In a food processor, pulse 3/4 cup hazelnuts until finely chopped.  Heat the heavy cream in a small saucepan over medium heat until bubbles appear at the edges. Remove from the heat and stir in the finely chopped hazelnuts. Let the mixture sit for 1 hour to flavor the heavy cream. Pour through a fine mesh strainer into a bowl to remove the hazelnuts.

For the filling, place the milk chocolate and semisweet chocolate in a heatproof bowl over a saucepan with 1 inch of simmering water. Stir occasionally until chocolate has melted. Remove from the heat and transfer to a large bowl. Stir in the cream. Roughly chop 1/4 cup hazelnuts and fold them into the mixture along with salt and vanilla extract. Carefully pour the filling into the baked tart shell. Chill in the refrigerator until set, about 3-4 hours.

Decorate the tart by pressing in the remaining hazelnuts. You can leave them whole or roughly chop them.

Store in the refrigerator covered. Let sit at room temperature 1-2 hours before serving.