Get a frying pan very hot, pop in your knob of butter, followed by the hearts, and fry them for 4 minutes, rolling them around occasionally. Apply a splash of balsamic vinegar and chicken stock, season, and let the hearts get to know the liquor for a couple of minutes. Place the hearts on the toast, leave the sauce on the heat to reduce for a moment, and pour over the toast and duck hearts. Eat. The hearts have an amazingly ducky quality.