Lift unsifted potato flour lightly into cup to measure.
Add baking powder and salt and sift together 3 times.
Beat egg whites until stiff and frothy but not dry.
Add sugar in 2 portions and beat just to blend after each addition.
Beat egg yolks thoroughly.
Fold into whites.
Add flour mixture gradually by sprinkling a tablespoon at a time over the top, and folding lightly and carefully until the flour is incorporated.
When all the flour is added, sprinkle a few drops of water at a time over the top and continue folding in until all the water is used.
Lift batter carefully by spoonfuls into heavily greased muffin pans.
Be sure to grease tins well as muffins stick easily.
Bake in a moderate oven (375° F.) for 18 to 20 minutes.
Serve warm with butter.