It sounds like Piece of Cake by Spuffy luvr But with an All Human cast
http://www.dark-solace.org/elysian/viewstory.php?sid=3770Buffy is devastated when Liam marries the woman he was cheating on her with - a woman with certain talents in the bedroom. Enter William. He knows exactly what her ex craves, and he's willing to teach Buffy the skills she lacks. But will it help her to win back her ex? Does she even want Liam back?
William has spent his adult life satisfying Drusilla's every whim and desire, but her latest whim involves getting married - and not to him. Alone now, he somehow finds himself Buffy's tutor. Although he enjoys their lessons, what he really wants is her. Will he be able to let her go when the time comes?