*Note* - sift the flour, or fluff it with a fork and then spoon it into your measuring cups for a softer, chewier cookie.
Pre-heat the oven to 350° F and line large baking pans with silpat mats or parchment paper (I prefer parchment)
Place the all-purpose flour, oats, baking powder, cinnamon, and salt in a medium bowl, whisk to combine, and set aside
Place the butter, brown sugar, and granulated sugar into the bowl of a stand mixer with the paddle attachment. Beat on medium speed until fluffy
Beat in the eggs one at a time, scraping the sides and beating thoroughly after each addition. Beat in the vanilla.
Slowly add the flour mixture, 1 cup at a time, beating on low and scraping the sides after each addition, and beat until just incorporated. Don't overmix.
Scoop the dough by heaping Tablespoons or using a cookie scoop and place about 2 inches apart
Bake 8 to 9 minutes for a chewier cookie, 10 - 11 minutes for a crispier cookie
Transfer to a cooling rack and allow to cool completely