Preheat oven to 350° F.
Grease a 9 by 13 inch baking dish with butter and set aside.
In a medium bowl, stir together the light brown sugar, all-purpose flour, cinnamon, and salt. Toss in the pecans. Pour the butter in and gently stir with a spoon until combined. Set aside.
Using a hand mixer, beat the sweet potatoes on high until they become light and smooth. Drop the speed to medium and mix in the butter, brown sugar, granulated sugar (Dixie Crystal), and cinnamon. Finish by adding in the eggs, heavy cream, vanilla, and salt.
Pour the sweet potato soufflé mixture into the prepared baking dish.
Evenly sprinkle the pecan mixture across the top of the sweet potato soufflé mixture.
Place in the oven on the center rack and bake for 50 minutes or until the center does not jiggle.
Carefully scatter the mini marshmallows on top of The Southern Sweet Potato Soufflé and broil for 1-2 minutes until lightly browned. Avoid walking away! They brown quickly!