Preheat your oven to 375°F and spray a 9x13 baking dish with nonstick cooking spray. 

Place cut and peeled sweet potatoes into a large stockpot and cover with water. Bring the water to a boil, and then cut the heat down to medium to simmer the potatoes for 15 minutes, or until they are fork tender.

Strain the potatoes and add them back into the pot or into a large bowl. Mash the potatoes with a potato masher, or use an electric hand mixer. Add the tablespoon of heavy cream and continue mixing until the sweet potatoes are a smooth consistency.

Now add in the granulated sugar and brown sugar, the softened butter, cinnamon, salt, and half of the pecans. Use a spoon to mix everything until it is fully combined. 

Pour the sweet potato mixture into the prepared baking dish. Top them with the mini marshmallows to cover the whole dish. Now sprinkle on the remaining chopped pecans.

Bake in the sweet potato casserole in the oven uncovered for 20-15 minutes or until the marshmallows are golden brown.