Stir together eggs, flour, and milk; cover and chill batter 8 hours.
Sprinkle roast with salt and pepper. Place roast on a foil-lined rack in a roasting pan.
Bake at 500° for 30 minutes. Reduce temperature to 350°, and bake 30 minutes. Increase temperature to 450°; bake 35 more minutes or to desired degree of doneness. Remove roast, and let stand 15 minutes before thinly slicing, reserving about 1/2 cup drippings in pan.
Spoon drippings into miniature muffin pan cups, filling one-fourth full.
Bake at 450° for 2 minutes or until thoroughly heated.
Stir chilled batter; spoon into miniature muffin pans, filling half full.
Bake at 450° for 9 minutes or until puffy and golden. Remove from oven, and make a well in center of pudding. Arrange Prime rib slices in centers; top with horseradish sauce or horseradish. Garnish, if desired. Serve warm.
*1/2 cup sour cream, 1 tablespoon prepared horseradish, 2 tablespoons lemon juice, and 1/4 teaspoon salt combined may be substituted for horseradish sauce.
Note: Freeze Yorkshire Puddings, if desired. Let thaw at room temperature 30 minutes; reheat at 400° for 5 to 7 minutes. Assemble as directed.