Anyone found that using a grainier organic brown sugar causes flat cookies?

Brown sugar also contains a certain amount of moisture, I wonder if variations in the water content might not be the reason. There are a variety of different methods to make brown sugar that result in variations in crystal size and moisture levels.

I wonder if that brown sugar was less dark than your usual brown sugar? Molasses (which is added back to white sugar to make brown sugar) is acidic, and that acidity contributes to fluffiness in cookies when it reacts with baking soda. If the organic brown sugar was more of a turbinado sugar as mrslarkin suggests, it may have been less acidic. The Food Lab at Serious Eats tested many chocolate chip cookie parameters like this if you're interested: http://sweets.seriouseats.com/2013/12/the-food-lab-the-best-chocolate-chip-cookies.html

Hi Lara. Hmmm. I haven't used organic brown sugar, but I'm guessing it might be of the very granular variety, almost like turbinado? I wonder if you'd get a better cookie result if you ran the sugar in the food processor a bit, to make a sort-of superfine brown sugar?