Boil sweet potato cubes for 15-20 minutes or until tender. Drain potatoes and mash until smooth. Stir in butter and set aside to cool.

Once sweet potato puree has cooled to room temperature transfer to the bowl of your mixer and add sugar, milk, eggs, nutmeg, cinnamon and vanilla. Whisk the mixture on medium speed until mixture is smooth. Pour filling into a 9 inch unbaked pie crust.

Bake at 425° for 15 minutes. Reduce heat to 350°; bake until set or a knife inserted in the center comes out clean, about 35-40 minutes longer. Cool completely, then refrigerate.

To make the topping, whisk together the egg whites, sugar, cream of tatar, and salt in a glass heat-proof bowl.

Place bowl over a small pot of boiling water. Make sure the boiling water does not touch the bottom of the bowl.

Whisk egg white mixture constantly until sugar has dissolved. Rub a bit of the mixture between to fingers to test. It should be smooth with no granules.

Transfer smooth egg white mixture to the bowl of your mixer fitted with the whisk attachment and beat on high until mixture has doubled in size and white and fluffy, about 6-8 minutes.

Stir in vanilla extract.

Spoon marshmallow cream onto pie. Refrigerate until set, at least 1 hour.

Just before serving, torch with a kitchen torch or place under the broiler set on high for 1-2 minutes until golden brown.