Preheat oven to 350 degrees F (175 degrees C).

Grease a 9x13 inch baking dish.

In a large bowl, mix together mashed sweet potatoes, melted butter, milk, sugar, vanilla extract, and eggs.

Spread sweet potato mixture into the prepared baking dish.

In a small bowl, mix together brown sugar and flour.

Cut in butter until mixture is crumbly, then stir in pecans.

Sprinkle pecan mixture over half of the sweet potatoes.

Bake for 25 minutes in the preheated oven.

Add marshmallows to the sweet potato mixture during the last 5 minutes of baking.

If desired it can be made with just the pecan topping or just the marshmallow topping.

If making ahead of time bake as directed for 25 minutes, and when about ready to serve warm in oven at 350 for 15 minutes then add marshmallows and bake for another 5 minutes.