In large bowl, beat the butter and sugar until fluffy. Add lemon zest, vanilla, and egg yolk. Stir well. Slowly add the hazelnuts and flour. Mix well. Shape dough into 2 balls and refrigerate for 1 hour. Preheat oven to 350 degrees. With a lightly floured rolling pin, roll out each ball of dough to 1/4 inch thick on a floured surface. Cut out cookies into a crescent shape. Place cookies on baking sheets about 1 inch apart. Bake until pale golden about 20 minutes. Allow the cookies to cool on the baking sheets then sprinkle with more sugar.