1. With a large, sharp knife, split the kidneys in half lengthwise without cutting all the way through. For the marinade, combine the chutney, prepared mustard, dry mustard, lemon juice, salt and cayenne pepper in a large mixing bowl, and stir until thoroughly mixed.
2. Add the kidneys and turn them about with a spoon to coat them evenly on all sides. Set the kidneys aside at room temperature to marinate for 1 hour, stirring them from time to time.
3. Remove the broiler pan from the oven and, using a pastry brush, coat it with softened butter. Setting the pan aside, preheat the broiler to its highest point. Then remove the kidneys from the marinade and spread them out flat on the pan, cut side up.
4. Broil the kidneys 3 inches from the heat for 3 minutes. Turn them over with tongs and broil those 3 minutes more. Spread about ˝ teaspoon of the marinade on each slice of toast, arrange the kidneys in pairs on the toast, and serve at once.
