This is kind of obvious but I thought I'd post it just in case anyone hasn't done this before. I seem to constantly run out of brown sugar but I have white sugar in my food storage to get thru and I always have molasses around because I use it in my bread. So I often make this homemade brown sugar when I need some for a recipe.
1 cup white sugar
1 tsp - 1 TB molasses
Beat well in a mixer. Add more or less molasses for a darker or lighter brown sugar.