Formula: Enchant Bracer - Greater Intellect (Classic - WoTLK)
The only enemy in the entire game that drops this recipe is the Vilebranch Shadowcaster located in Jintha'alor of the Hinterlands. This location is filled with elite trolls and as a whole is one of the locations I recommend for farming Mageweave Cloth; if you'd like to read more check out my Jintha'alor Mageweave Cloth Farming Location.
In addition to the Mageweave Cloth that you'll get from farming here another valuable item that these trolls drop is Wildvine. It's used in some very good crafting recipes in Tailoring as well as recipes that are added later in the game during Zul'gurub.
Basically how this area works is, each time you go up a floor (sometimes two floors) the types of enemies you fight change. The Shadowcasters that you need are on the fourth floor of the area, so when you first enter keep running up stairs until you start seeing these Shadowcaster enemies.
Respawn time is kind of slow in this area, if it's just you and you're level 60 you'll be able to keep the entire camp locked down since there aren't that many enemies for you to hunt here. For this reason too I would recommend you come here during an off hour, this will help make sure you don't run into any groups doing the quests too.
As usual, players with Herbalism and or Mining will see a small spike in how much money they can make here. For Herbalists, throughout the majority of Jintha'alor the only herb you'll find is Purple Lotus, which isn't worth very much. However, if you're a stealth class then while you're waiting for respawns you can head up to the top of Jintha'alor into one of the two caves. Inside of these caves you'll find Ghost Mushroom, which is one of the most valuable herbs in the game due to how rare it is.
Miners will find Mithril Ore throughout the area as well as sometimes a Truesilver Deposits. Be sure to keep an eye out for Solid Chests as well, they're pretty common throughout Jintha'alor - I think I have seen one spawn on each of the different levels of the area.
Your biggest money maker at this location though by far will be Mageweave Cloth and Wildvine, both of which are dropped by every troll here. These items will sell on the Auction House for quite a bit of gold and make it actually worth your time to farm here.
Items of Interest:
Mageweave Cloth
Wildvine
Mithril Ore
Purple Lotus