In a medium bowl, whisk together the flour, cinnamon, baking soda, and salt. Set aside.
In a large bowl, use a hand mixer or a stand mixer with a paddle attachment. Beat the butter, brown sugar, and granulated sugar together on medium-high speed. Mix until everything is well combined and creamy, which should take about 3 minutes.
Add the eggs, molasses, and vanilla and beat on high speed until combined, about 1 minute. Scrape down the sides and bottom of the bowl and beat again as needed to combine.
Add the dry ingredients to the wet ingredients and mix on low speed until combined. Beat in the oats and butterscotch chips. Dough will be thick and sticky.
Cover and refrigerate the dough for at least 45 minutes (and up to 4 days).
If you chill the dough for more than a few hours, let it sit at room temperature for at least 30 minutes. This will help soften the dough before you scoop out the cookie balls and bake them.
Preheat oven to 350°F (177°C). Line baking sheets with parchment paper or silicone baking mats. Set aside.
Use a medium cookie scoop to scoop out the cookie dough. Each scoop should be about 2 tablespoons (40g) of dough. Place the dough balls 3 inches apart on the baking sheets.
Bake for 13–14 minutes or until lightly browned on the sides. The centers will look very soft.
Remove the cookies from the oven and let them cool on the baking sheet for 5 minutes, then transfer them to a cooling rack to cool completely.
Optional: While the cookies are still warm, I like to press a few more butterscotch chips into the tops and give them a light sprinkle of flaky sea salt.
Cookies stay fresh covered at room temperature for up to 1 week.