01 - Preheat the oven to 350°F. Line two baking sheets with parchment paper.
02 - In a medium bowl, whisk together gluten-free flour, hazelnut flour, baking soda, and salt until evenly blended.
03 - In a large bowl, beat the softened butter and ¾ cup granulated sugar with a mixer until light and fluffy, approximately 2 minutes.
04 - Mix in the egg, lemon zest, lemon juice, and vanilla extract until fully incorporated and smooth.
05 - Gradually add the dry ingredients to the wet mixture, mixing on low speed just until combined. Do not overmix.
06 - Gently fold the chopped toasted hazelnuts into the dough until evenly distributed.
07 - Scoop 1 tablespoon of dough per cookie, roll into balls, and coat each ball in the extra granulated sugar.
08 - Place cookies 2 inches apart on prepared baking sheets. Gently flatten each ball with your fingers or the bottom of a glass.
09 - Bake for 10 to 12 minutes, or until edges are set and bottoms are lightly golden brown.
10 - Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.