Cut a sheet of parchment paper long and wide enough to cover the bottom and sides of an 8-inch square pan with 2 inches of overhang on all sides. Crumple up the parchment and straighten it out half a dozen times to soften it, so that it will fit into the corners without sharp edges. Line the pan with the parchment paper across the bottom and up the sides, pressing creases at the bottom and top edges.
In a bowl of a food processor fitted with a metal blade, process 1 cup of the hazelnuts, the flour, and 6 tablespoons of the sugar until the nuts are finely ground, about 2 minutes. Add the butter and process until the dough gathers around the blade, about 1 minute.
Scrape the dough into the prepared pan, press it with your hands into an even layer, and smooth the top with a small offset spatula.
Coarsely chop the remaining 1 and 1/2 cups of hazelnuts with a knife, aiming for mostly halves and leaving some whole.
In a medium saucepan over medium-high heat, bring the cream and the remaining 6 tablespoons of sugar to a boil, stirring frequently. Cook until it is thick enough to coat a spoon (running a finger down the back should leave a clear track), 3 to 5 minutes. Take the pan off the heat, immediately add the chopped nuts, and stir to coat them evenly. Use a spoon to distribute the nut mixture evenly over the dough, pressing lightly on the nuts with the back of the spoon to level them. Sprinkle the fleur de sel evenly over the surface. Freeze the dough, uncovered, until firm, 30 to 40 minutes.
Meanwhile, set a rack in the middle of the oven and preheat the oven to 350°F. Line a baking sheet with parchment paper.
Using the parchment overhang as handles, lift the dough out of the pan and transfer it to a cutting board. Using a large heavy knife, cut the dough into 5 equal strips in each direction to make 25 squares. Arrange as many cookies as you can fit on the prepared baking sheet, leaving about 1 1/2 inches all around them. Keep the rest of the unbaked cookies in the refrigerator.
Bake until the tops are caramelized and the hazelnuts are golden brown, 26 to 28 minutes Set the sheet on a wire rack to cool for 10 minutes, then transfer the cookies directly onto the rack to cool completely. Repeat with the remaining cookies.
The cookies will keep in an airtight container at room temperature for up to 5 days.