Here’s the good news: light brown sugar and dark brown sugar are basically sugar twins—not identical twins, but close enough to share a wardrobe. They’re both just white granulated sugar with molasses added back in. The only difference? Molasses content. Dark has about twice as much, which gives it a deeper, more caramel-ish flavor.
So, if you’re in a pinch and only have light brown sugar, go ahead and use it. Your cookies won’t stage a rebellion. I promise.