When I first arrived in France in 1996, brown sugar was non-existent. But I was in the South where the little red gems known as bettrave don't exist. Then I moved to Lille where I discovered tarte à la cassonade AND vergeoise. They even make a tarte au sucre. Oh the choices!
Over the past few years, soft brown sugar commercially available at most major supermarkets. Called saveur vergeoise, it is actually cane sugar with caramel color and glucose syrup for moisture. It works well in all recipes I’ve tried. Brown sugar as we Americans know it is sugar with the molasses added back in, not water and color. Beurk!
I prefer to use sucre graeffe which is beetroot sugar rather than the soft sugar available at the supermarket. It has a strange, vegetable smell which is surprising at first but it works as a one to one replacement for brown sugar (both light and dark) in all of my recipes. However, it may be difficult to find because it is a very local product in Belgium and Northern France. There is also a dark brown sugar called candico which is made from sucre candi and not beetroot. It is closer to saveur vergeoise than sucre graeffe. Be aware that cassonade and vergeoise are not the same thing. Cassonade is the equivalent to raw cane sugar and does not contain moisture where vergeoise is moist.
A little beetroot sugar is nothing a little chocolate chip cookie can't handle...