Once slightly cooled, mash the sweet potatoes with butter and salt
Grease a 9×13 inch baking dish with butter or coconut oil and fill with mashed potatoes.
In a double boiler or small pan with water and a glass bowl on top, whip the egg whites with a hand blender until fluffy.
Add stevia and turn on the heat.
Use a whisk to whip the egg white and stevia mix as it heats. Whisk for about 20 minutes (or less) until it thickens.
It will start to take on the consistency of melted marshmallows… when it is thick enough, spread over the sweet potatoes and put it in the oven under broil for a minute or two to brown.