On the Islands, when one is completed the Brewing Another Batch repeatable Quest or the similar initial non-repeatable one called Potion Making 101, but before you actually turn those into the Cauldron, if you talk to Uhura it appears that there is another Quest 'available' called 'The Next Batch Is Brewing'. But try as you might to clock on it, you can't. Well actually the Icon is for a completed Quest but you can't really turn those in there, it has to be the Cauldron.