Pulse the oats briefly in a food processor until broken but not powdered.

In a medium bowl, whisk together the dry ingredients: pulsed oats, gluten-free flour, cornstarch, baking powder, baking soda, cinnamon, and salt. Set aside.

In a stand mixer, cream butter with brown and granulated sugars for 2–3 minutes until smooth and fluffy.

Add eggs one at a time, mixing well after each addition. Stir in the vanilla.

Combine the wet and dry ingredients, mixing briefly to avoid overmixing; leave a few dry streaks.

Fold in chocolate chips and raisins until evenly distributed.

Cover and refrigerate the dough for 2 hours or up to 1 day.

Preheat the oven to 350°F (177°C) and line baking sheets with parchment paper.

Scoop 3-Tablespoon balls of dough and place them on the baking sheets, spacing them out.

Bake one sheet at a time for 10–12 minutes until the centers are almost set, optionally pressing extra chips/raisins on top before baking.

Cool cookies on the baking sheet for 2–3 minutes before transferring them to a wire rack to cool completely.