"Note: It is really important that the butter isn't too hot when you start adding other ingredients to it. If you don't have a thermometer to test temperature, let the butter sit for the full 10 minutes. It should be at warm room temperature but not hot at all. Also, if you are curious about subbing light brown sugar, do so at your own risk. I have only ever used dark brown sugar - it helps the chewy factor be even more, well, chewy...."