Preheat the oven to 350 degrees F. Line a large baking sheet with parchment paper or a Silpat baking mat. Set aside.
In a medium bowl, whisk together the flour, baking soda, cinnamon, and salt. Set aside.
Use a stand mixer or hand mixer to beat the butter on medium speed for about 30 seconds. Add in the sugars and beat until smooth, about 1 minute, scraping down the sides of the bowl with a spatula as necessary.
Add the egg and vanilla extract; mix until combined. With the mixer off, add all of the dry ingredients. Mix on low until just combined, don’t over mix.
Stir in the oats and butterscotch chips.
Use a cookie scoop or spoon to scoop the dough into balls. Place on the prepared baking sheet, about 2 inches apart.
Bake for 10 to 12 minutes or until cookies are slightly golden around the edges but still soft in the middle. They will finish baking as they cool. Remove the pan from the oven and sprinkle cookies with flaky sea salt. Let the cookies cool on the baking sheet for 3 minutes and then transfer to a cooling rack to cool completely.