Make Ahead and Storage: The cookies can be stored in an airtight container for up to 1 week. Pro Tip: To help keep the cookies flat, gently bang the baking sheets on the oven racks when you rotate them. Why It Works: This cookie's chewy perfection relies on a just-right combo of moisture, spread, and rise: Brown sugar contributes moisture, granulated sugar and butter promote spread, and baking powder and baking soda give a little lift.