Dice the unsalted butter and place it in a light-bottomed saucepan over medium heat. Allow the butter to melt, then continue cooking with frequent stirring. The butter will start to bubble and boil; watch closely and stir constantly to prevent burning. After a couple of minutes, as the bubbling subsides and the butter turns a deep golden color with a nutty aroma, and the bottom layers darken, immediately remove from heat. Transfer the browned butter to a heatproof bowl and let it cool at room temperature for about 20 minutes. For quicker cooling, you can use a bowl with a wide bottom. Personally, I like to scrape the brown deposits from the bottom of the saucepan back into the butter—they add extra flavor to the cookies!