This recipe is slightly adapted from America's Test Kitchen. There are very few ingredients; the cookies are basically oats held together with a lot of butter, and made crisp with a combination of white and light brown sugar. —Merrill Stubbs
about 3 dozen small cookies
unsalted butter, at room temperature
Heat the oven to 350° F and arrange a rack in the middle of the oven. Line two baking sheets with parchment paper.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon. Set aside.
In the bowl of a standing mixer, or using handheld beaters, cream the butter and the two sugars until light and fluffy, 3 to 5 minutes. Scrape down the sides of the bowl, add the egg and beat until incorporated, about 30 seconds. Scrape down the bowl and do the same with the vanilla.
On low speed, add the flour mixture and beat until just combined. Remove the bowl from the mixer and fold in the oats by hand, mixing just until there are no remaining flour pockets.
Form balls out of tablespoon-sized pieces of dough and place them 2 inches apart on the parchment-lined cookie sheets. Bake one sheet at a time for about 15 minutes, rotating once for even cooking. Let the cookies cool completely on the baking sheets for maximum crispness. The cookies will keep in an airtight container for a week.