The Legend:
Sometime before 1731, the remains of the 62-year-old Countess Cornelia di Bandi of Cesena were found on the floor of her bedroom by her maid. Her body had been reduced to ashes; all that survived were her stockinged legs and, on the floor between them, her head. Her bed was undamaged, with the covers thrown back, and it was assumed that she was consumed suddenly as she was walking from the bed to the window.
The Countess was not given to intoxication, which confounded those who believed spontaneous combustion to be caused by an overindulgence of drink.

Variations & Theories
The earliest account of this case I have found is from Charles Dickens of all people; in the preface to his novel Bleak House, he defends his belief in Spontaneous human Combustion by citeing details from three real life cases; the Countess Cornelia's death is one of them. Here's his account of it:

"There are about thirty cases on record, of which the most famous, that of the Countess Cornelia de Bandi Cesenate, was minutely investigated and described by Giuseppe Bianchini, a prebendary of Verona, otherwise distinguished in letters, who published an account of it at Verona, in 1731, which he afterwards republished at Rome."

And this is all he has to say on the matter, which, at the very least, says the event occured previous to 1731. Naturally, I will attempt to locate the source he mentioned.
A second early source is identified by Joe Nickell in his book, Secrets of the Supernatural. His source is an article from a 1746 issue of Gentleman's Magazine (#16, pg. 368), and from it he sets the date of the event as sometime within the year 1731 and gives an alternative possible name for the Countess, namely 'Baudi' instead of 'Bandi.' He reports the remains as consisting of a pile of ashes and the Countess' legs and arms, all in relatively untouched condition, and her head, which was found between her legs; burned away were the brain, part of the cranium, and her entire chin. The ashes "left in the hand a greasy and stinking moisture," soot floated in the air in the room, "and from the lower part of the window, trickled down a greasy, loathsome, yellowish liquor with an unusual stink." On the floor was an empty oil lamp covered with ashes; Nickell takes this as clear evidence that the Countess had fallen on the lamp. Naturally, I will also hunt down a copy of this article to double-check this information, though it is interesting to note that Nickell does not explain how the Countess' head came to lay between her relatively untouched legs.

Sources:

Bleak House, Charles Dickens, 1853 Bradbury & Evans [London], 1972 Everyman's Library. The Encyclopedia of Unsolved Mysteries, Colin and Damon Wilson, 1988 Contemporary Books, Inc. Secrets of the Supernatural, Joe Nickell (with John F. Fischer), 1988 Prometheus Books.

See Also:

General Article: Spontaneous Human Combustion