Remove butter from fridge 30 minutes in advance so it reaches room temperature.
Using a hand mixer, cream together the butter, brown sugar, and granulated sugar until light and creamy, at least 3 minutes. Beat in the egg and vanilla extract until smooth.
To make oat flour, blend up regular oats in a blender or food processor until they resemble flour. Measure to get a leveled 1/4 cup. Add to the same bowl on top of the wet ingredients: oat flour, baking soda, salt, cornstarch, cinnamon, and old-fashioned oats. Beat until ingredients are well combined.
Add the flour (measure by spooning the flour in the measuring cups and leveling). Mix until just combined. Cover dough tightly and chill for 1 hour. These cookies will spread without being chilled.
Preheat oven to 350°F. Scoop out dough balls (if you have a food scale, do cookie balls about 1.7 ounces in size; this will yield 15 cookies).
Line a sheet pan with parchment paper or a baking liner. Place dough balls (no more than 9 cookies at a time) on the sheet pan. Return the tray of cookies to the fridge for 10 minutes.
Bake 7–9 minutes or until lightly browned at the edges (even if the center looks a little under-done—they cook a bit more after being pulled out of the oven). These cookies set up and become extremely delicious and chewy if they’re slightly underbaked. Err on the side of slightly underbaking these cookies for soft, chewy, and tender cookies! Add a sprinkle of sea salt if desired to the cookies. Cookies are best enjoyed within 2–3 days
Notes
Note 1: I love the hint of cinnamon in these cookies, but you can leave it out if you prefer. For more of a spiced cookie, add 1/8 teaspoon of ground nutmeg and ground cloves in addition to the cinnamon.Note 2: It’s easy to make your own oat flour. Just add old-fashioned oats to a blender and process until it’s the texture of flour. Stir it around to be sure all the oats are ground.Storage: Store baked cookies in an airtight container at room temperature for up to a week or freeze in a sealed bag. Freeze dough balls separately and bake from frozen, adding a few extra minutes.