For the brain dumplings as a soup garnish, sauté the shallots or onion in butter, poach the brains in lightly salted water (or milk/water).
Let cool and cut into small cubes, stir butter and yolk until fluffy, add breadcrumbs, flour and finally chopped brains. Season to taste.
Chill the mixture for about 1 hour, then form small dumplings.
Put into boiling salted water (or boiling water from the brains) and cook for about 6 to 8 minutes below boiling point, depending on size.