3 cups raw, skin-on hazelnuts, divided
3/4 teaspoon cinnamon, divided
2 cups flour
1 cup plus 3 tbsp. cold butter, cubed
3/4 cup granulated sugar, divided
1/2 teaspoon salt, divided
7 tablespoons maple syrup
3 tablespoons packed light brown sugar
1. Preheat oven to 350°. Roast hazelnuts on a rimmed baking sheet until light golden, about 18 minutes. Rub the warm nuts in a kitchen towel to remove half of skins. Finely chop 1/2 cup nuts in a food processor. Reduce oven temperature to 325°.
2. Put chopped nuts, 1/2 tsp. cinnamon, the flour, 1 cup butter, 1/2 cup granulated sugar, and 1/4 tsp. salt in a large bowl. Blend on low speed with a mixer until well blended.
3. Grease a 9-in. square baking pan, line with foil (let it hang over edges), then grease foil. Press dough evenly into bottom of pan. Bake until golden and firm, about 45 minutes. Meanwhile, very roughly chop half of remaining nuts to make 1 cup.
4. Bring remaining 1/4 tsp. cinnamon, 3 tbsp. butter, 1/4 cup granulated sugar, 1/4 tsp. salt, the maple syrup, and the brown sugar to a boil in a small saucepan, whisking frequently. Remove from heat and stir in 1 cup whole hazelnuts and the chopped nuts (2 cups total; you may have leftover nuts). Pour over crust and spread evenly. Bake until set when pan is tilted, about 18 minutes.
5. Let cool on a rack. Lift foil with cookie to a board. Cut cookie into 16 squares, then cut each square in half diagonally.