In the bowl of a stand mixer, whisk together flour, nuts, cocoa, sugar and salt. With paddle attachment, pastry blender, or using fingers, cut butter into dry mixture until pea-sized crumbs are formed. Slowly add ice water and vanilla, just until dough comes together (not sticky). Add 1 tsp additional water at a time if dough is too dry.
Remove 300 gr. dough and reserve for another purpose (enough for three 5-inch tarts). Flatten dough between pieces of plastic wrap or plastic wrap and parchment, and roll out to 1/8-inch thick. Chill on sheet pan in refrigerator for about 10 to 15 minutes, just to firm up the dough a little before placing in the pan. Remove the parchment from one side of the dough, retaining the plastic wrap on the other side. Roll out circle of dough about 1½ inches larger than the pan.
Grease the tart pan well, and carefully flip the dough into the prepped pan. Use the plastic wrap to support the edges of the dough as you fit the dough down into the bottom, and up against the sides of the pan. Trim excess dough by trimming at the top of the pan. Chill slightly before filling.
In a medium, heavy bottom sauce pan, heat milk and chocolate. In a small bowl, whisk egg yolks with sugar until completely mixed. In a very small bowl, mix salt with cornstarch, add to egg mixture and mix completely. Slowly add the hot milk to the egg mixture, in half cup increments, mixing thoroughly between each addition. Return to sauce pan and slowly bring to just bubbling. Whisk constantly for about 5 to 6 minutes on med/low heat. Take off the heat and whisk for another minute, adding the vanilla and butter. Mix one last time, and allow to cool before filling shells, whisking once in a while to prevent a skin forming on top.
Preheat oven to 425°, with rack in lower third of the oven. Whisk cooled chocolate cream before filling. Bake 25 to 30 minutes at 425°, until crust has darkened and small cracks appear in the edge.
Do not cut pie until totally set and cool; use serrated knife to cut for best results. Chill slightly to set up if cutting from a large pie. Serve with a drizzle of caramel, and a scattering of chopped nuts.
Chef’s Note: Toast hazelnuts in 350° oven on parchment lined sheet pan for 6 or 7 minutes. Pour from sheet pan into a paper bag and shake vigorously to remove the skins. Toast extra nuts for topping for the tart; chop coarsely instead of finely.
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