Coat an 8-inch square baking dish with cooking spray and set aside.
Bake the potatoes until they are soft 45 to 60 minutes.
When cool enough to handle, peel the potatoes and place in a mixing bowl.
Mash the potatoes, using a fork or, if the potatoes are fibrous, use a food processor.
Add the butter and mix until blended.
Mix in the pineapple, stevia, salt and pepper.
Spread the potato mixture in the prepared baking dish, smoothing the top.
Cover with marshmallows, arranging them 1/2-inch apart in one layer. (If desired, the dish can be covered and refrigerated overnight; bring to room temperature before baking.)
Bake the casserole for 20-25 minutes, or until the marshmallows are brown and crusty.
Nutrition information has been calculated using an autogenerating online calculation and is intended for informational purposes. For accurate nutritional information, you should calculate the nutritional information with the actual ingredients used.