Preheat the oven. Preheat your oven to 375°F and line a baking sheet with parchment paper. This helps prevent the cookies from sticking and encourages even baking.
Cream butter and sugar. In the bowl of a stand mixer fitted with the paddle attachment, beat the room-temperature butter and brown sugar for 2–3 minutes, or until light, fluffy, and well combined. Scrape down the sides of the bowl as needed to ensure everything mixes evenly.
Add the eggs and vanilla. Add the egg, egg yolk, and vanilla extract. Mix until smooth and fully incorporated, scraping down the sides of the bowl as needed.
Combine the dry ingredients. In a separate medium bowl, whisk together the gluten-free all-purpose flour, old-fashioned rolled oats, quick-cooking oats, baking soda, baking powder, cinnamon, and salt. Whisking helps distribute the leavening agents and spices evenly throughout the dough.
Mix the dough. Gradually add the dry ingredients to the stand mixer. Mix on low speed just until combined, being careful not to overmix. The dough should be thick and slightly sticky.
Add the raisins and pecans. Fold in the raisins and chopped pecans using a spatula or mix briefly on low speed until evenly distributed throughout the dough.
Chill the dough. Cover the bowl and refrigerate the dough for 30 minutes. Chilling helps the oats and gluten-free flour absorb moisture, firms up the butter, and prevents the cookies from spreading too much during baking.
Scoop the cookies. Use a 2 tablespoon cookie scoop or 3 tablespoon to portion the chilled dough onto the prepared baking sheet, leaving about 2 inches between each cookie to allow for spreading. Keep the dough portions evenly sized so the cookies bake at the same rate.
Bake the cookies. Bake at 375°F for 9–12 minutes, or until the edges are lightly golden and the centers look just set. The cookies may appear slightly soft in the middle when you remove them from the oven, but they will continue to firm up as they cool.
Cool and serve. Let the cookies rest on the baking sheet for 5–10 minutes before transferring them to a wire rack to cool completely. This allows them to set without breaking while keeping the centers soft and chewy.