1. Sift together flour and salt. Place in bowl; make a well, and place eggs in center. Slowly whisk eggs into flour mixture until a paste forms. Gradually whisk in 1/2 cup milk.
Gradually whisk in remaining z cups milk. Cover with plastic; chill in the refrigeratorat least 4 hours, or overnight.
2. When roast is finished, set oven at 425 degrees. When pan has been deglazed, pour 1/4 cup reserved pan drippings into roasting pan. Heat pan and drippings until very hot, about 5 minutes. Remove batter from refrigerator, and shake or whisk well; quickly pour into hot pan. Cook until crisp and golden, 20 to 30 minutes. Serve warm with Prime Rib.