Well in advance (anywhere from 2 hours to a week) of making the lamb, combine the butter, yolks, bread crumbs, rosemary, mustard seed and Dijon mustard in the bowl of a food processor and pulse until thoroughly mixed. Line a loaf pan with plastic wrap. Using a spatula, spoon the mustard crust mixture into the pan, smooth the surface, and place in the refrigerator. Allow to chill until solid.
Preheat an oven to 350 degrees.
Season the lamb with the salt and pepper. Heat the olive oil in a heavy skillet over medium heat for 2 minutes. Place the racks, meat side down, in the skillet, and brown for 2 to 3 minutes to a crispy brown coating. Turn over. Cook 1 minute more on the bone side. Remove from the saute pan or skillet, and place in a roasting pan. Place the racks in the oven and cook 8 to 10 minutes for medium rare.
While the lamb is roasting, remove the mustard crust by lifting the plastic wrap out of the loaf pan. You should now have a slab of seasoned mustard crust. Cut the loaf of herb butter into 1/2 inch thick slices. Refrigerate them until the lamb has cooked for 8 to 10 minutes. Remove the lamb from the oven, lay the slabs of seasoned mustard crust on the meat side of the lamb, and press the butter with your finger tips to help it to adhere to the meat. Return the lamb to the oven for 5 minutes more to allow the butter to form a crust. Remove the racks from the oven and allow to cool for 5 minutes before slicing into individual chops or 4rib rack portions.