Preheat oven to 375°F. Wash morels in 3 or 4 waters to make sure that all sand and grit are removed from the little honeycombs. Place morels in 1cup boiling water for 1 minute, then drain.
Carefully remove stems. Chop stems very fine and mix with ground veal. Soak the roll in milk.
Cook onion in half of the butter, covered, for 10 minutes. Add the mixture of veal and mushroom stems, the chopped parsley, salt and white pepper. Mix it well and cook, covered, over very low heat for 10 minutes.
Squeeze the soaked roll, and force it through a sieve or purée in a blender. Mix it with the egg yolks and then mix it well with the cooked stuffing.
Carefully stuff the morels. Spread 2tablespoons of remaining butter in a pottery baking-serving dish. Place the morels in the dish. Sprinkle with sour cream and bread crumbs. Place a tiny bit of remaining butter on top of each mushroom.
Bake in the preheated oven for 15 to 20 minutes, or until golden brown. Serve in same dish.