I have not used brown sugar in CP soaps, yet, but I do use white sugar (1 TBSP ppo). I add it to the water, dissolve and then add my lye. I would imagine it would work the same way. It will boost the lather some. But I wonder if the soap would end up being brown. Might be, but I am thinking that if it is scented with Vanilla, it would not matter. Hmmmm, I think I need to add this to my "To Do List". Would love to see pictures, please.