Prepare a medium-hot charcoal fire or preheat a gas grill for 15 minutes or medium-high. Mix together the breadcrumbs, parsley, garlic, pecorino, and pepper to taste. Add enough olive oil to make a paste. Place the slices of veal between 2 pieces of wax paper and flatten until they are about 1/16 inch thick. Place a thin slice of butter on every rectangle of meat, then cover with a slice of mortadella, trimming to the same size if necessary. Cover the mortadella with some of the breadcrumb mixture and lightly sprinkle with salt. Roll up and secure with toothpicks. Double skewer the roll-ups. Slide the roll-ups onto the skewers so they fit tightly, using 2 roll-ups per double skewer. Place the skewers on the grill and cook until golden brown, 8 to 10 minutes on each side. Baste frequently with butter.