Fry the bacon until crisp and put it in a warm place. Pour off most of the fat leaving just enough to brown the liver. Brown it quickly over a brisk flame, reduce the heat and cook to the degree of doneness you like. Remove to a warm place while you make a sauce by adding the butter, 1/2 cup or so of beef stock, and Madeira to taste to the pan. Simmer 5 minutes until somewhat reduced. Arrange the liver on hot plates. Pour on the sauce, then crumble the bacon on it and garnish with chopped parsley. Good served with asparagus or spinach.