01 - Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.
02 - Whisk together all-purpose flour, ground hazelnuts, baking powder, and fine sea salt in a medium bowl.
03 - In a large bowl, beat unsalted butter and granulated sugar using an electric mixer or whisk until the mixture is light and creamy.
04 - Mix in vanilla extract and egg yolk until thoroughly incorporated.
05 - Gradually stir the dry ingredients into the butter mixture until a soft dough forms.
06 - Scoop tablespoon-sized pieces of dough and roll into balls. Position on prepared baking sheet, leaving about 2 inches between each.
07 - Gently flatten each dough ball with the back of a spoon. If desired, sprinkle with chopped hazelnuts and coarse sugar.
08 - Bake for 12 to 14 minutes, until the edges of the cookies are lightly golden.
09 - Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack until fully cooled.