14tablespoons(198g)butter (I use salted), softened but still slightly cool
¼cup(53g)packed light brown sugar
2 ½cups(250g)old-fashioned rolled oats
Adjust an oven rack to the middle position and heat the oven to 350 degrees. Line large, rimmed baking sheets with parchment paper and set aside.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
In a large bowl (of a stand mixer or with a handheld electric mixer), beat the butter, granulated sugar and brown sugar together until just combined, about 20 seconds. Increase the mixer speed to medium and continue to beat until light and fluffy, about 1 minute longer. Scrape down the sides of the bowl with a rubber spatula.
Add the egg and vanilla and beat on medium-low until well mixed, about 30 seconds. Scrape down the sides of the bowl again. Add the flour mixture and mix until barely incorporated, 10-20 seconds. It's ok if there are a few dry spots.
Gradually add the oats and mix until well-combined, about 30 seconds to 1 minutes. If needed, give the dough a final stir with a wooden spoon to ensure that no flour pockets remain and that the ingredients are evenly distributed.
Scoop out about 2 tablespoon-sized mounds of dough and roll them to form balls. Place the cookies about 2 1/2-inches apart on the baking sheet(s) - about 8 cookies per sheet. They will spread quite a bit. Lightly press each cookie to about 3/4-inch thickness (I found after baking one sheet of these that I didn't need to press them at all so use your best cookie judgment).
Bake 1 sheet of cookies at a time until the cookies are golden brown, edges are crisp, and centers are still very slightly soft, 13 to 16 minutes. Cooling the cookies completely on the baking sheet will yield crispier, more perfect cookies.
Oats: don't substitute quick oats in this recipe; you'll hate me because the cookies won't look like the ones in the pictures. It's old-fashioned rolled oats all the way here.