01 - Preheat the oven to 350°F. Line two baking sheets with parchment paper.
02 - In a medium bowl, whisk together the gluten-free flour blend, baking powder, and fine sea salt until evenly distributed.
03 - In a large bowl, beat the softened butter and granulated sugar using a hand mixer or stand mixer until light and fluffy, approximately 2 minutes.
04 - Add the egg, lemon zest, fresh lemon juice, and vanilla extract to the butter mixture. Mix until well combined and smooth.
05 - Gradually add the dry ingredient mixture to the wet ingredients, mixing just until a soft dough forms. Do not overmix.
06 - Gently fold the roughly chopped pistachios into the dough until evenly distributed.
07 - Scoop tablespoon-sized portions of dough and roll into balls. Place them 2 inches apart on the prepared baking sheets. Flatten each ball slightly with your fingers or the bottom of a glass.
08 - Bake for 10 to 12 minutes, or until the edges are just golden. Allow cookies to cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
09 - Whisk powdered sugar with fresh lemon juice until smooth and pourable. Drizzle generously over the cooled cookies. Garnish with extra chopped pistachios and lemon zest if desired. Let the glaze set before serving.