Preheat your oven to 350 degrees.
Whisk together the cooked (and mashed) sweet potatoes, white sugar, vanilla extract, beaten eggs, & heavy cream. Spread into a casserole dish.
In a separate bowl, combine the softened butter, brown sugar, flour, pecans, & vanilla extract. I like to use my hands for this part, it makes it so much easier in my opinion. Crumble the topping all over the sweet potatoes.
Bake for 30 minutes. Sprinkle on marshmallows & return to the oven for 3-5 minutes, until your marshmallows are golden.