After taking the cooked potatoes out of the oven, lower the temperature to 350° F. Cut a 1/2 inch slice off the top of each potato and scoop out the flesh to leave a 1/4 inch thick shell.
Mash the flesh, with the butter, mustard and egg yolks. Stir in the cheese, ham, chives, and some salt and pepper.
Whisk the egg whites until fairly stiff and fold into the potato mixture.
Pile back into the shells and bake for 20 minutes until puffed and lightly browned.
Serve hot with a crisp green leafy salad.Servings: 4