Preheat oven to 350 degrees Fahrenheit. Line a baking sheet with parchment paper or a silicone baking mat.
In the bowl of a stand mixer, beat the butter, brown sugar, and sugar until light and fluffy, about 3 minutes.
Add the egg and yolk to the butter mixture and beat until incorporated.
Add the flour, oats, baking soda, cinnamon, and kosher salt. Mix until incorporated and the mixture forms a cookie dough.
Use a small cookie scoop to scoop 6 cookies onto the prepared sheet pan. Bake for 11 minutes or until the edges are golden brown and the centers look underdone.
Let the cookies cool on the sheet pan for at least 5 minutes before transferring to a cooling rack. Repeat with the remaining cookie dough.