1. Heat oven to 350 degrees F. In a large bowl, beat butter, both sugars, and vanilla. In a medium-size bowl, whisk together flour, hazelnuts, baking powder, and salt.
2. Beat the egg yolk into butter mixture. Stir in flour mixture to form a stiff dough. Gather dough together.
3. Roll 1 tablespoon dough into a ball. Place on ungreased baking sheet and make an indentation in the center with your thumb. Repeat with remaining dough.
4. Bake at 350 degrees F for 15 minutes, or until lightly browned. Cool completely on wire rack.Drizzle and Filling:
5. In small bowl, stir confectioners' sugar and 1-1/2 teaspoons water until smooth. Transfer to a small plastic bag. Snip one corner from bag; drizzle over top of cookies. Let dry.
6. Once dry, fill centers with 1/4 teaspoon jam. To keep more than 2 days, store in unfilled airtight container.