Cut twelve slices of bread; trim the crusts so the slices will be of even size. Cut out the centers from one-half the slices, leaving a wall of one inch. Toast the solid slices. Brush the untoasted edge of the bread with a little white of egg, lay on the rims and put them in the oven to toast on the upper side. Pick the sweetbreads apart, after they are carefully cooked, rejecting the membrane. Slice the mushrooms. Cut the chicken into dice. Put the butter into a saucepan, add the mushrooms, toss for a minute until the mushrooms are slightly softened, then add the flour, mix, and add the milk, salt and pepper. Cover this on the back part of the stove for ten or fifteen minutes until the mushrooms are cooked; then add the meat. Stand this over hot water ten or fifteen minutes. The toast should now be done and crisp. Arrange each canapé on a square of lace paper on an individual heated dish, put the mixture in the center, garnish with the yolk of the eggs pressed through a sieve. Garnish the very top with a little chopped truffle or a little chopped parsley. These are the handsomest of all hot canapés, and while they are usually served following the soup at dinner, they may be used for the main course at a ladies' luncheon, or at a supper.