Vanillekipferl (Vanilla Crescent Cookies)

0 Prepare Vanilla Sugar: Blend the powdered sugar with the scraped out seeds of the vanilla bean in a food processor or bowl. Set aside.
2 Grind the Nuts: In the same food processor, pulse the walnuts (or almonds/hazelnuts) until very finely ground—slightly coarser than almond meal. Add a tablespoon or two of flour to prevent sticking.
4 Make the Dough: Sift together flour, a pinch of salt, and (if using) cornstarch. Add ground nuts, softened butter chunks, and ¾ cup vanilla sugar mixture. Mix with a hand or stand mixer (with a paddle attachment) on low speed until combined, then increase to medium and knead until a smooth dough forms. 
6 Finish kneading by hand if necessary. If the dough feels sticky, chill it for about [timer minutes=30]30 minutes to 1 hour[/timer] until firm.
7 Shape the Cookies: Divide the dough into 10 portions. Roll each portion into a 10-inch rope and cut into six equal pieces. If necessary, refrigerate for [timer minutes=30]30 minutes[/timer] until firm. Shape each piece into a crescent about 2–3 inches long, tapering the ends (watch video). Place on a parchment-lined baking sheet, spacing 1–2 inches apart.
9 Chill and Bake: Refrigerate the shaped crescents for [timer minutes=15]15–30 minutes[/timer] to help them hold their shape (this prevents spreading too much while baking). Preheat oven to 350°F (175°C) and then bake the cookies for [timer minutes=12]12 to 14 minutes[/timer], until the tips are just golden and small cracks form on top. Let them cool slightly on the tray—these cookies are fragile when hot.
11 Dust with Vanilla Sugar: While still warm, dust generously with remaining vanilla-powdered sugar. Let cool completely before storing.