Preheat the oven to 350 degrees F (180 C). Line baking sheets with parchment paper or silicone baking mats and set aside.
In a medium bowl, whisk together the flour, cinnamon, baking powder, baking soda, and salt; set aside.
Using a stand mixer or electric hand mixer, beat together the butter, granulated sugar, and brown sugar on medium speed until the mixture is light and fluffy. Add the egg and vanilla and beat on medium-low to combine.
Scrape down the sides of the bowl, then add the flour mixture and beat just until incorporated. (Try not to overmix; a few streaks of flour are OK.) Gradually add the oats and mix until evenly combined.
Scoop out mounds of dough approximately 2 tablespoons each and roll into balls. Place the cookies about 2 inches apart on the baking sheets; they will spread out.
Bake for 12-15 minutes, until the cookies are golden brown with crisp edges and very slightly soft centers. If baking more than one pan at a time, rotate them midway through the bake time to ensure the cookies brown evenly. Cool cookies completely on the baking sheets, then store tightly covered at room temperature.
Notes
Cookies will keep well at room temperature at least 3-4 days.
Do not sub quick cooking for the old-fashioned rolled oats! They do not hold up as well when mixed into the dough and baked, and the end result will NOT be the same!