Preheat your oven to 400°F (205°C). Wash sweet potatoes thoroughly and pierce several times with a fork. Place directly on the oven rack and bake for 45-55 minutes until completely tender when pierced with a knife.
Once the sweet potatoes are done, reduce oven temperature to 350°F (175°C). Lightly grease a 9×13-inch baking dish with butter or cooking spray.
When cool enough to handle, peel sweet potatoes and place the flesh in a large mixing bowl. Add softened butter and sugar, then beat with an electric mixer until smooth and fluffy.
Beat in eggs one at a time, then add vanilla extract, milk, cinnamon, nutmeg, and salt. Continue mixing until completely incorporated and no streaks remain.
In a separate bowl, combine brown sugar, flour, chopped pecans, melted butter, and cinnamon. Mix until the ingredients are evenly moistened and form a crumbly texture.
Pour the sweet potato mixture into the prepared baking dish, smoothing the top with a spatula. Sprinkle the pecan topping evenly over the surface.
Bake at 350°F for 30-35 minutes until the center is just set and the topping is golden brown and fragrant.
Let the soufflé rest for 5-10 minutes after baking before serving.