Directions

1. Preheat oven to 350°F with racks in upper and lower thirds of the oven. Line 2 sheet pans with parchment paper. (Note that baking 2 pans at a time and switching them halfway through baking saves time. Cookies bake the most evenly cooked 1 pan at a time in the center of the oven, but the recipe will take longer to make.)
2. In a large bowl, combine oats, flour, baking soda, salt, and cinnamon. Set aside.
3. In the bowl of an electric mixer, using the paddle attachment, beat butter, brown sugar, and granulated sugar on medium-high speed until light and creamy, 3-4 minutes. Stop to scrape down the bowl as needed so mixture is uniform. Add egg, egg yolk, and vanilla one at a time, mixing after each on medium speed until blended, and scraping down the bowl as needed.