Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius) and grease a 2-quart baking dish.
In a large mixing bowl, combine the mashed sweet potatoes, granulated sugar, melted butter, eggs, vanilla extract, and milk.
Whisk the mixture until smooth and pour it into the prepared baking dish.
In a medium bowl, combine the brown sugar, flour, softened butter, and chopped pecans.
Mix the topping ingredients with a fork or pastry cutter until crumbly.
Sprinkle the topping evenly over the sweet potato base.
Bake for 45 minutes until the topping is golden brown and the souffle is set.
Allow to cool for 5 to 10 minutes before serving.