Cream The Butter & Sugars. Combine the softened butter, brown sugar, and white sugar in a mixer. Cream for 2-3 minutes, or until the mixture is lighter in color and light and fluffy in texture.
Add Eggs One At A Time, beating to incorporate each egg before adding the next one.
Add Vanilla, and mix until it's combined.
Start On The Dry Ingredients. Scrape down the edges of your mixer and add your gluten-free flour, rolled oats, cornstarch, baking soda, salt, and cinnamon. Mix until *almost* combined. (There should still be a few dry spots left.)
Add Butterscotch Chips & Mix just until the flour is incorporated and the butterscotch chips are evenly distributed in the batter. Try not to overmix the dough.
Cover & Chill At Least 2 Hours. DO NOT SKIP this step! Cover your dough (I transfer mine to an airtight container) and chill at least 2 hours, up to one day.
Preheat & Prep. When you're ready to bake the cookies, preheat the oven to 350 degrees F. and line a baking sheet with parchment paper. Remove the dough from the fridge 15-20 minutes in advance of baking so the dough is soft enough to scoop.
Scoop & Bake. Use a large (3 Tbsp.) cookie scoop to scoop 6 cookies onto the baking sheet at a time. Bake 10-12 minutes at 350 degrees F. or until the cookies are just turning golden around the edges and they look a bit underdone in the center. Remove from the oven & sprinkle with flaky salt, if desired.
Cool & Transfer. Allow the cookies to cool 2-3 minutes on the baking sheet before transferring to a cooling rack to continue cooling. The residual heat from the pan and cookie will help set the centers of your cookies.
Repeat & Eat! Then, repeat this baking process with the rest of your dough. Enjoy the cookies fresh, store leftover cookies at room temperature for 2-3 days, or freeze 6=8 weeks.