01 - Set oven to 350°F (175°C) and line two baking sheets with parchment paper.
02 - In a medium mixing bowl, whisk together gluten-free flour blend, cocoa powder, baking soda, and sea salt until evenly incorporated.
03 - In a large bowl, beat softened unsalted butter, packed brown sugar, and granulated sugar until mixture is creamy and light, about 2 minutes using an electric mixer or whisk.
04 - Mix in the large egg and pure vanilla extract with the creamed mixture until thoroughly combined.
05 - Gradually stir the dry ingredient blend into the wet mixture, mixing just until a soft dough forms.
06 - Gently incorporate chopped dark chocolate, toasted hazelnuts, and crisped rice cereal into the dough using a spatula.
07 - Scoop tablespoonfuls of dough and place onto prepared baking sheets, spacing them 2 inches apart to allow for spreading.
08 - Transfer baking sheets to the oven and bake for 10 to 12 minutes, until cookie edges are set but centers remain slightly soft.
09 - Allow cookies to cool on baking sheets for 5 minutes before transferring them to a wire rack to cool completely.