Preheat the oven to 375 degrees F. Line two baking sheets with parchment paper.
Whisk together the flour, kosher salt, baking powder and baking soda in a medium bowl. Set aside.
Beat the butter, chocolate-hazelnut spread, brown sugar and granulated sugar in the bowl of a stand mixer fitted with the paddle attachment on medium-high speed until light and fluffy, about 4 minutes. Beat in the egg and vanilla. Reduce the speed to low and slowly add the flour mixture until just combined. Fold in the chocolate chunks and hazelnuts by hand.
Drop 12 heaping tablespoonfuls of dough onto each of the prepared baking sheets, about 2 inches apart. Roll the dough into balls. Sprinkle each dough ball with fleur de sel. Freeze until firm, about 20 minutes.
Bake, rotating the pans halfway through, until the cookies are golden around the edges but still soft in the middle, about 15 minutes. Let cool on the baking sheets for 10 minutes, then remove to wire racks to cool completely, about 30 minutes.