Place sweet potatoes in a large stockpot; cover with 1 inch of water. Bring to a boil over medium heat and cook until fork tender, about 20 minutes. Drain, allow to cool and remove skins.
Preheat the oven to 350 degrees F (175 degrees C). Grease or butter a 2-quart casserole dish.
Place potatoes in a mixing bowl; beat with an electric mixer on low speed until potatoes begin to break up. Increase speed to medium high and blend until smooth.
Reduce speed to low and add sugar, milk, butter, vanilla, eggs and salt. Mix well.
Allow any potato fibers to remain on the beater and remove. Pour sweet potato mixture into the casserole dish.
To make the topping: Mix brown sugar, flour, butter, and pecans together in a small bowl. Sprinkle mixture over sweet potato mixture.
Bake in the preheated oven until the center of the soufflé is set, about 40 minutes.