Wash the potatoes and line a pan with parchment paper or foil.

Preheat the oven to 400F and bake them for about an hour or until they are soft and oozing sugar.

Remove them from the oven and reduce the oven temperature to 350F.

While the potatoes are cooling, melt the butter.

In a large bowl, combine the butter, sugar, milk, egg, and extract, whisking vigorously.

When the sweet potatoes are cool enough to handle, skin them and add them to the bowl along with the salt.

Mash everything together until it is blended but still just a little chunky.

Butter a casserole dish and fill it with mashed potatoe mixture.

Bake for 20 minutes at 350F.

When it is done, add the marshmallows and bake for an additional 5 minutes.

If needed, broiler can be used to get a toasted finish on the marshmallow topping.