How to Make It
Heat oven to 350° F. Spread the hazelnuts on a large rimmed baking sheet and toast in oven, tossing occasionally, until fragrant, 10 to 12 minutes. Rub the warm nuts in a clean dish towel to remove the skins (discard the skins).
In a food processor, finely grind ½ cup of the hazelnuts. Add the crushed cookies and butter and pulse until moistened. Press the mixture into the bottom and up the sides of a 9-inch removable-bottom fluted tart pan.
Place on a rimmed baking sheet and bake until dry, 10 to 12 minutes (if the crust puffs up during baking, gently press it down with the back of a spoon). Let cool in the pan on a wire rack.
Meanwhile, in a medium saucepan, bring the cream just to a boil. Remove from heat and whisk in the chocolate until smooth. Pour into the cooled tart shell. Refrigerate until set, about 1 hour.
Chop the remaining ¼ cup nuts and sprinkle on the tart.
Sprinkle with the salt just before serving. For the easiest slicing, use a thin, sharp knife, wiping it clean and running it under hot water between slices.