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°.
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.
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.
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.
Let cool on a rack. Lift foil with cookie to a board. Cut cookie into 16 squares, then cut each square in half diagonally.
Extra-Toasty Hazelnut Shortbread Bars: Follow directions for Hazelnut Shortbread Bars. Keep temperature at 350° throughout recipe. In step 3, bake crust until very deep golden, 45 minutes to 1 hour. In step 4, reduce the baking time to about 15 minutes.
Chocolaty Hazelnut Shortbread Bars: Follow directions for Hazelnut Shortbread Bars. After spreading nut topping on the crust in step 4, scatter 1 cup (5½ oz.) roughly chopped bittersweet chocolate on top, then bake.
Fruity Hazelnut Shortbread Bars: Follow directions for Hazelnut Shortbread Bars. In step 4, stir 1 cup tart or sweet dried cherries or cranberries into nut topping.
*Hazelnuts from Oregon--where virtually all the U.S. crop grows--taste much sweeter and fresher than imports. Find them in bulk bins in grocery stores (check the country of origin) and at freddyguys.com, hazelnuthill.com, or yournw.com?
Note: Nutritional analysis is per bar.