Thanfully, gravity takes over, and the excess fat drips off into the tray below (I don't use foil). Butter is merely a good method of 'securing' the garlic, rosemary et al to the leg of lamb when sealing and afterwards roasting it.
How do you do yours?
Plenty of incisions into the raw meat - smother the whole lot in butter which has chopped garlic, rosemary and anchovies mashed in. No salt. Seal it, slow roast it.