In a large mixing bowl (use a stand mixer fitted with a paddle attachment, or a handheld mixer), add your softened butter and packed brown sugar. Beat on medium-high speed for approximately 3 minutes until the mixture becomes light, fluffy, and noticeably pale in color. You should be able to see air bubbles incorporated throughout.
This step is essential—you're aerating the mixture, which helps the cookies rise evenly and creates that tender crumb. If your butter is cold, the mixture will look grainy and refuse to fluff up. Room temperature really matters here. I learned this the hard way after baking in a chilly kitchen one January morning!