For the souffle: Preheat the oven to 350 degrees F. Grease a 2 1/2-quart baking dish with butter.
Poke the sweet potatoes with a fork and bake on a foil-covered baking sheet until they are soft, 1 hour. When cool enough to handle, peel the potatoes, place the flesh in a large mixing bowl and mash until very smooth. Add the eggs, sugar, butter, vanilla, milk and salt. Combine well with an electric mixer or hand mixer. Turn the mixture into the baking dish.
For the topping: In a medium bowl, stir together the pecans, brown sugar, flour and butter until thoroughly combined. Spoon the mixture over the sweet potatoes, making an even layer. Bake the casserole until slightly browned, 40 minutes. Let the casserole sit for 5 minutes before serving.