In lieu of cupcake Monday today, here is a bit of cool information I found in a magazine that you might find helpful.
This is a question from Cuisine at Home magazine:
Besides color, what's the difference between light and dark brown sugar?
The biggest difference is favor. Both light and dark brown sugar are refined white sugars that have molasses added to them for color and flavor.
Dark brown sugar can contain nearly double the amount of molasses of light brown sugar. That extra molasses can add a lot of extra flavor and moisture to the recipes you're using it in.
Because the additional molasses adds moisture dark brown sugar yields a moister end product than light brown sugar. Most of the time this isn't an issue, so you should be able to us whichever you have on hand. That said, you might want to be careful swapping then in moisture-and density- sensitive recipes, like cakes.
*I have to admit, I never really pay much attention to dark or light brown sugar, I just buy what's in the bag at the grocery store. After reading this article I rushed to see what it is I'm buying, "golden brown" is what the packaging says. Interesting. I think I'm safe.*