Begin by browning 1/2 cup of unsalted butter. For instructions on browning butter, please see above. Place browned butter in the refrigerator to cool for 15-20 minutes (butter does not need to re-solidify).
Preheat oven to 350 degrees and line a baking sheet with parchment paper.Add (cooled) browned butter and 1/2 cup softened unsalted butter to the bowl of a stand mixer. Mix on low speed until the butters have combined and then increase the speed to medium-high. Beat the butter until it is fluffy and light, about 3 minutes. Add both of the sugars and mix on medium-high speed for another 2 minutes.
Then add the eggs, one at a time, mixing in between each addition. Add the vanilla and mix. Then scrape down the sides of the bowl and then mix for another minute.
In a separate, medium bowl, combine flour, baking powder, baking soda, salt, and cinnamon. Whisk together until combined.
Add the flour mixture to the wet ingredients, mixing on low until combined. Remove the bowl from the stand. Add the old fashioned oats and stir in by hand. Finally, add the butterscotch chips and stir until all mixed in.
Scoop rounded tablespoons of dough out onto the prepared baking sheet. Cook 8-11 minutes until the cookies are golden around the outside and look slight undercooked in the middles. Do NOT overcook. Optional: When the cookies are right out of the oven, give them a small sprinkle of flaky sea salt.
Let the cookies sit on the hot baking sheet for 10 minutes to let them set. Then remove them to a cooling rack to finish cooling completely. Repeat steps for the remaining cookie dough.Enjoy!