01 - Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper and set aside.
02 - In a medium bowl, whisk together the gluten-free flour blend, baking soda, and salt until evenly distributed.
03 - In a large mixing bowl, combine the melted butter, brown sugar, and granulated sugar. Whisk until smooth and creamy, about 1 minute.
04 - Add the egg and vanilla extract to the butter-sugar mixture, whisking until fully incorporated and the mixture appears glossy.
05 - Gradually fold the dry ingredient mixture into the wet ingredients using a spatula, mixing until just combined. Be careful not to overwork the dough.
06 - Gently fold in the dark chocolate chips and roughly chopped walnuts until evenly distributed throughout the dough.
07 - Using a tablespoon or small cookie scoop, drop rounded mounds of dough onto the prepared baking sheets, spacing them approximately 2 inches apart to allow for spreading.
08 - Bake for 11 to 13 minutes, or until the edges are golden brown and the centers appear just set. Rotate baking sheets halfway through for even baking.
09 - Allow the cookies to rest on the baking sheets for 5 minutes to firm up, then transfer to a wire cooling rack to cool completely before serving.