Adjust oven rack to middle position and heat oven to 350 degrees. Line 3 large (18- by 12-inch) baking sheets with parchment paper. Whisk flour, baking powder, baking soda, and salt in medium bowl.

In standing mixer with paddle, beat butter and sugar at medium-low speed until just combined, about 20 seconds. Increase speed to medium and continue to beat until light and fluffy, about 1 minute longer. Scrape down bowl with rubber spatula. Add egg and vanilla and beat on medium-low until fully incorporated, about 30 seconds. Scrape down bowl again. With mixer running at low speed, add flour mixture and mix until just incorporated and smooth, 10 seconds. With mixer still running on low, gradually add oats and mix until well incorporated, 20 seconds.

Divide dough into 30 equal portions, each about 1-1/2 tablespoons, then roll between palms into balls. Place cookies on prepared baking sheets, spacing them about 21/2 inches apart, 10 dough balls per sheet. Using fingertips, gently press each dough ball to 1/2 to 3/4-inch thickness.

Bake 1 sheet at a time until cookies are deep golden brown, edges are crisp, and centers yield to slight pressure when pressed, 13 to 16 minutes, rotating baking sheet halfway through. Transfer baking sheet to wire rack; cool cookies completely on sheet.

Note: It may sound counterintuitive, but doubling the usual amount of leavener (we used both baking powder and baking soda) in oatmeal cookies is the key to crispiness. The amplified dose creates big bubbles that first help the dough rise, then combine and burst, resulting in a flat cookie.

This last batch, I used all white sugar instead of the original 1 cup white plus 1/4 cup brown. Also used 12 tablespoons butter instead of 14. PERFECT! Got good spread, nice crisp and delicious. The above recipe reflects the changes