In the bowl of your stand mixer, combine room temperature butter, both sugars, and salt. Beat at medium speed for 6-8 minutes until fluffy and pale, pausing halfway to scrape down the bowl and beater with a flexible spatula.
In a separate bowl, combine gluten-free flour, cornstarch, oat flour, baking soda, and cinnamon.
Measure out your gluten-free oats and add-ins, setting them aside in a separate bowl.
With the mixer running on low, add the egg, allowing it to fully incorporate. Pause your mixer to scrape down the sides.
Slowly add the flour mixture to the butter mixture while the mixer is on low speed. Mix until just combined.
Add your gluten-free old fashioned oats and add-ins, mixing at a low speed until just combined.
Transfer the batter to a Tupperware container and let the dough rest in the fridge for at least 4 hours, preferably 12 hours or longer.
When ready to bake, allow the dough to come to room temperature until scoopable. Use a 2-tablespoon cookie scoop (#20 yellow scoop) to portion the cookie dough. This recipe yields 10-12 cookies.
Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
Arrange 6 cookie dough balls on the baking sheet, leaving room for spreading. Place the remaining dough in the fridge or freezer for future use.
Bake the cookies for 10 minutes, then rotate the baking sheet. Continue baking for an additional 6-8 minutes or until the cookies are lightly golden and beginning to set.
Remove from the oven and, if desired, use a round cookie cutter slightly larger than the cookie to shape them into perfect circles.
Sprinkle with flaky sea salt and allow the cookies to cool for 5-10 minutes on the baking pan before transferring them to a cooling rack.