01 - Preheat oven to 350°F. Line a baking sheet with parchment paper.
02 - In a medium bowl, whisk together all-purpose flour, ground hazelnuts, baking powder, and salt until thoroughly combined.
03 - In a large bowl, beat softened butter and granulated sugar with an electric mixer or whisk until mixture is light and creamy.
04 - Blend in vanilla extract and egg yolk, mixing until evenly incorporated.
05 - Gradually incorporate dry ingredients into the creamed mixture, mixing just until a soft dough forms.
06 - Scoop out one tablespoon-sized portions of dough, roll into balls, and arrange on prepared baking sheet, maintaining 2-inch spacing between each.
07 - Gently flatten each dough ball with the back of a spoon. Sprinkle with chopped hazelnuts and coarse sugar if desired.
08 - Bake in preheated oven for 12 to 14 minutes, until cookie edges are lightly golden.
09 - Let cookies cool on baking sheet for 5 minutes, then transfer to a wire rack to cool completely.