Preheat the oven to 350°F and butter a 9x13 baking dish.
Boil the sweet potatoes in a large pot until tender, then drain well.
Place the hot sweet potatoes in a large bowl and add 4 tablespoons melted butter, 1/2 cup brown sugar, milk, eggs, vanilla, salt, and cinnamon.
Mash or blend the mixture until smooth and creamy.
Spread the sweet potato mixture evenly into the prepared baking dish.
In a separate bowl, combine the chopped pecans, 1 cup brown sugar, and flour.
Stir in 4 tablespoons melted butter until the topping is crumbly.
Sprinkle the pecan topping evenly over the sweet potato layer.
Bake for 30 to 35 minutes, or until the topping is golden and the edges are bubbly.
Let the casserole rest for 10 minutes before serving.