For the crust, heat the oven to 350ºF. Spread the hazelnuts on a rimmed baking sheet and toast in the oven until lightly browned and skins start to blister, 10 to 12 minutes. Rub the hot toasted hazelnuts in a clean kitchen towel to remove skins (it’s ok if not all the skins come off), then let cool completely. Transfer the cooled nuts to the bowl of a food processor and pulse until the nuts resemble a coarse meal.
In a large bowl, whisk together the hazelnuts, flour, and salt. Using your fingertips, blend in the butter until incorporated, leaving some roughly pea-sized lumps. Working with 1 tablespoon at a time, stir in 2 to 4 tablespoons ice water, just until the dough holds together.
Place the dough in the center of the 9-inch tart pan with a removable bottom. Using your fingers and a flat-bottomed measuring cup, spread and push the dough to evenly cover the bottom and sides of the pan. Prick the dough all over with the tines of a fork. Freeze until firm, at least 30 minutes or overnight wrapped tightly in plastic.
For the filling, in a medium bowl, stir together the maple sugar, brown sugar, egg, flour, butter, and 1/3 cup cream until well combined.
To assemble, increase the oven temperature to 375ºF with the rack in the middle. Remove the tart shell from the freezer and bake until the dough is just starting to brown, about 20 minutes. Pour the filling into the hot crust and continue baking until the crust is golden and the edges of the filling are just set (the center will be fairly jiggly), 35 to 38 minutes more. Cool the tart in the pan on a rack for 1 hour (the filling will continue to set as it cools), then remove the sides of the pan.
At the table, just before serving, pour the remaining 1/2 cup chilled cream over the top of the tart, then cut into wedges.The tart, without the cream poured over the top, can be kept, covered and refrigerated, for up to 1 day. Bring it to room temperature, then pour on the cream just before serving.