Brown sugar is a very moist ingredient. Unfortunately, it loses it's moisture really easily and becomes hard and solid. I have a quick and simple solution if the brown sugar in your gourmet baking mix has hardened.
First, remove the ingredients that are on top of the brown sugar. Chances are, if the brown sugar is hard it'll be easy to separate. If the brown sugar is too hard to pour out of the jar, use a knife to carefully break it up into manageable chunks. Separate the hardened pieces into a microwavable dish and microwave at 10 second intervals until it's soft and manageable. If you have particularly stubborn sugar, add a couple of drops of water.
You can now proceed with the recipe! Moving along quickly will ensure the brown sugar doesn't re-harden.