Preheat the oven to 325 degrees F.
With a rolling pin, roll out each bread slice until thin. Cut out bread with plain round cutter 1 1/2 inches in diameter and flatten again with a rolling pin. Oil small muffin tins and place the bread rounds inside. Bake until golden. Store the croustade shells covered in a dry area.
Reduce the cream by half and reserve until needed. Saute the chicken in 1 tablespoon oil until half cooked. Add the prosciutto and continue to cook until is thoroughly heated and chicken is thoroughly cooked. Add the cream and black pepper, to taste; bring to a simmer. Add a little hot cream to the egg yolk to temper and add to pot, being careful not to boil. Add the cheese, basil, and parsley. Adjust seasoning. Fill the croustade shells; sprinkle with a little additional grated Parmesan and brown lightly under a broiler or salamander. Serve warm.
Recipe courtesy of Garde Manger, John Wiley and Sons, Inc., 2000