Step 1

Preheat oven to 350°. Toast hazelnuts on a rimmed baking sheet, tossing once, until fragrant and slightly darkened, 8–10 minutes; let cool. Using a kitchen towel, rub hazelnuts together to remove most of the skins (don’t worry about some stubborn bits remaining).

Step 2

Pulse flour, sugar, salt, and ¾ cup hazelnuts in a food processor until the consistency of coarse meal. Reserve remaining ½ cup hazelnuts for filling. Add butter to dry ingredients and pulse until there are only a few pea-size pieces of butter remaining.

Step 3

Transfer mixture to a large bowl; drizzle 3 Tbsp. ice water over and mix, adding another splash or so of water if needed, just to bring dough together.

Step 4

Using your fingers, press dough evenly 1½–2" up the sides and then into the bottom of pan; compact and smooth with a flat, straight-sided measuring cup or glass. Chill 20 minutes.

Step 5

Bake crust until golden but not totally baked through, 15–20 minutes. Transfer to a wire rack and let cool 10 minutes.

Step 6

Meanwhile, bring cream of tartar, 1 cup granulated sugar, and 2 Tbsp. water to a boil in a medium saucepan over medium heat, stirring until sugar dissolves. Increase heat to medium-high and cook, swirling pan occasionally (do not stir), until caramel is a deep amber color, 8–10 minutes. Remove from heat. Whisking constantly, carefully add cream (mixture will be extremely hot and will bubble vigorously); whisk until smooth. Let caramel cool slightly in pot.

Step 7

Whisk pumpkin purée, 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.

Step 8

Bake tart, rotating halfway through, until filling is set around edges and center barely jiggles, 30–35 minutes. Transfer to a wire rack and let cool in pan.

Step 9

Meanwhile, coarsely chop reserved ½ cup hazelnuts and cook in a small skillet over medium heat with remaining ¼ cup sugar and 1 Tbsp. water, stirring constantly, until sugar caramelizes and evenly coats hazelnuts. Transfer caramelized hazelnuts to a sheet of parchment paper and let cool completely. Coarsely chop.

Step 10

Scatter caramelized hazelnuts over tart and serve with whipped cream.

Step 11

Do Ahead: Tart (without nuts) can be made 2 days ahead; cover and chill. Hazelnuts can be caramelized 2 days ahead; store airtight at room temperature.