In a large pot, cook sweet potatoes, in boiling water, 45 minutes to 1 hour or until tender.
Let cool to touch; peel and mash.
Pre-heat the oven to 350°F.
In the bowl of an electric mixer, combine sweet potatoes, sugar, eggs, vanilla, milk and 1/2 cup butter.
Beat, at medium speed, until smooth.
Spoon into a lightly greased 12-inch x 8-inch x 2-inch baking dish.
In a small bowl, combine brown sugar, pecans, flour, 2 tbsp. butter and nutmeg; mix well to combine.
Sprinkle mixture over casserole; add miniature marshmallows as last layer, if using.
Place in the pre-heated oven and bake for 30 minutes or until marshmallows are puffed and golden brown.
Notes
Instead of using fresh sweet potatoes, I generally use two 40-ounce cans of yams; there is no difference in the quality nor taste of the Sweet Potato Casserole.