1. Preheat your oven to 375°F (190°C).

2. Wash and peel the sweet potatoes. Cut them into small chunks and place them in a large pot of boiling water. Cook for about 15 minutes, or until the sweet potatoes are soft.

3. Drain the sweet potatoes and mash them with a potato masher or fork until smooth.

4. In a separate bowl, cream together the softened butter, brown sugar, and white sugar. Add in the mashed sweet potatoes and mix well.

5. Beat in the eggs, one at a time, and then add in the vanilla extract, milk, cinnamon, nutmeg, and salt. Stir until all ingredients are well incorporated.

6. Gradually mix in the flour, making sure there are no lumps in the batter.

7. Grease a 9×9 inch baking dish and pour the sweet potato mixture into it.

8. Sprinkle the chopped pecans on top of the sweet potato mixture.

9. If desired, add the marshmallows on top of the pecans. This will give the souffle a nice, gooey topping.

10. Bake for 25-30 minutes, or until the top is golden brown and the souffle has risen.

11. Serve hot and enjoy your delicious sweet potato souffle!