http://en.wikipedia.org/wiki/Yorkshire_pudding
History
When wheat flour began to come into common use for making cakes and puddings, cooks in the north of England devised a means of making use of the fat that dropped into the dripping pan to cook a batter pudding while the meat roasted. In 1737 a recipe for 'A dripping pudding' was published in "The Whole Duty of a Woman":[1]
Make a good batter as for pancakes; put in a hot toss-pan over the fire with a bit of butter to fry the bottom a little then put the pan and butter under a shoulder of mutton, instead of a dripping pan, keeping frequently shaking it by the handle and it will be light and savoury, and fit to take up when your mutton is enough; then turn it in a dish and serve it hot.
Similar instructions were published in 1747 in ‘The Art of Cookery Made Plain and Simple’ by Hannah Glasse under the title of 'Yorkshire pudding'. It was she who re-invented and re-named the original version, called Dripping Pudding, which had been cooked in England for centuries, although these puddings were much flatter than the puffy versions known today.[2]
"A Yorkshire pudding isn't a Yorkshire pudding if it is less than four inches tall, says the Royal Society of Chemistry"; true Yorkshire people would disagree - the modern trend has been to aim for Puddings of almost Souffle proportions - but back in 1737 they had ranges and not modern fan assisted ovens of today.[3]
The Yorkshire pudding is a staple of the British Sunday lunch and in some cases is eaten as a separate course prior to the main meat dish. This was the traditional method of eating the pudding and is still common in parts of Yorkshire today. Because the rich gravy from the roast meat drippings was used up with the first course, the main meat and vegetable course was often served with a parsley or white sauce.
It is often claimed that the purpose of the dish was to provide a cheap way to fill the diners - the Yorkshire pudding being much cheaper than the other constituents of the meal - thus stretching a lesser amount of the more expensive ingredients as the Yorkshire pudding was traditionally served first.[4]