In large bowl, whisk together eggs, milk and salt. Sift in the flour in three stages, each time whisking until flour disappears before adding in more flour. Let rest at room temperature for 30 minutes and up to 3 hours.
When the prime rib roast is finished, spoon out 3 tablespoons of beef fat. Turn oven to 450F.
Stir 1 tablespoon of the beef fat into the batter.
In a muffin pan, fill each cup with 1/2 teaspoon of the remaining beef fat. When oven has reached temperature, place muffin pan with fat into oven for 3 minutes until smoking hot.
Carefully take out pan (careful! it's hot!) and pour batter into each cup, filling to 2/3 full. Immediately return to oven and bake for 20 minutes. Do not open oven door during baking, or the Yorkshire Puddings will collapse. Reduce heat to 350F and bake an additional 10 minutes until golden brown.
Remove from oven, pierce each Yorkshire Pudding with toothpick to allow steam to escape and prevent them from collapsing.
Cooks Illustrated. Oh and pssst....I've even made them with bacon drippings when I wasn't in the mood for making prime rib....delish! but don't tell anyone I said that!