A great article from the Food Lab on the science of Yorkshire puddings:
http://www.seriouseats.com/2015/12/food-lab-yorkshire-pudding-popover-best-method-science.html
And his recommended recipe for making the best and fluffiest:
http://www.seriouseats.com/recipes/2015/12/the-best-yorkshire-pudding-popover-recipe.html