This comes from the English side of the family. I can trace it back to my grandmother, Emma Rowland Beard, but I imagine she got it from her predecessors. (One of them was executive chef at the Statler Hotel in Buffalo, NY. I believe he was William Rowland. I have inherited his long carver for steamship rounds, whole hams, and such.)
It's a rather simple recipe. I have it memorized (and passed it along to "my son the chef") as 1 dry, 2 wet. My grandmother used a manual rotary egg beater, my mother used a Mixmaster.
Yorkshire Pudding
1 cup flour
1 tsp salt
2 eggs
2 cups milk.
Mix the above and let stand to come to room temperature, with occasional additional mixing.
When the roast is out of the oven and resting, pour a good amount of accumulated fat from the roast into a baking pan of suitable size. Put the pan in the over and set the oven temperature to 400 degrees. When hot, CAREFULLY pour the batter into the pan. Bake until puffed and deep chestnut brown.
Remove from oven, cut into appropriate sized pieces and serve on a warmed platter. (The family tradition includes showing off the beautifully baked and puffed pudding to those who are dining before cutting it up.)