To make the crust, preheat oven to 350F. Toast hazelnuts on a rimmed baking sheet, tossing once, until fragrant and slightly darkened, about 8 - 10 minutes. Cool, then using a kitchen towel, rub hazelnuts together to remove most of the skins (don't worry about stubborn skin parts remaining). Pulse flour, sugar, salt, and 3/4 hazelnuts in a food processor until it reaches the consistency of coarse meal. Reserve remaining 1/2 cup hazelnuts for filling. Add butter to dry ingredients and pulse until there are only a few pea-sized pieces of butter left
Transfer crust mixture to large bowl. Drizzle 3 tbsp ice water over and mix, adding another splash or so of water just to bring dough together. Using your fingers, press dough evenly 1.5-2 inches up the sides of a greased springform pan. Compact and smooth with a flat, straight-sided measuring cup or glass. Chill 20 minutes
Bake crust until golden but not totally baked through, 15 - 20 minutes. Transfer to a wire rack and let cool 10 minutes
To make the filling, bring cream of tartar, 1 cup sugar, and 2 tbsp water to boil in a medium saucepan over medium heat. Stir until sugar dissolves. Increase heat to medium-high and cook without stirring until caramel is a deep amber color, 8 - 10 minutes. Remove from heat, and, whisking constantly, add cream. Whisk until smooth. Let caramel cool slightly in pot
Whisk pumpkin puree, brown sugar, flour, ginger, cinnamon, and salt in a large bowl. Gradually add caramel and eggs, whisking until well blended. Scrape filling into prepared crust
Bake tart, rotating halfway through, until filling is set around the edges and center barely jiggles. Transfer to a wire rack and let cool in pan
Meanwhile, coarsely chop reserved hazelnuts and cook in a small skillet over medium heat with remaining 1/4 cup sugar and 1 tbsp water, stirring constantly, until sugar caramelizes and evenly coats hazelnuts. Transfer hazelnuts to a sheet of parchment and let cool completely. Coarsely chop. Scatter hazelnuts over tart and serve with whipped cream