Preheat your oven to 400°F (200°C). Place the peeled and chopped sweet potatoes on a baking sheet. Roast for 25-30 minutes, or until fork-tender. This method enhances their natural sweetness and reduces moisture compared to boiling. Alternatively, you can boil the sweet potatoes in a large pot of salted water until tender, about 15-20 minutes. Drain well.

Transfer the cooked sweet potatoes to a large mixing bowl. Add the 1/2 cup melted butter, granulated sugar, milk, beaten eggs, vanilla extract, cinnamon, nutmeg, and salt. Mash thoroughly with a potato masher until smooth and creamy. For an extra smooth consistency, you can use an electric mixer. Taste and adjust seasoning if necessary.

In a separate medium bowl, combine the 1/2 cup packed light brown sugar, 1/2 cup all-purpose flour, and 1 cup chopped pecans. Cut in the 4 tablespoons of cold unsalted butter using a pastry blender, a fork, or your fingertips until the mixture resembles coarse crumbles.

Lightly grease a 9x13-inch baking dish. Spread the mashed sweet potato mixture evenly into the prepared dish.

Sprinkle the pecan streusel topping evenly over the sweet potato mixture.

Bake in the preheated oven at 375°F (190°C) for 30-35 minutes, or until the topping is golden brown and the casserole is bubbly around the edges. Keep an eye on the topping to prevent burning; if it starts to brown too quickly, you can loosely tent it with aluminum foil.

Remove from the oven and let it rest for 5-10 minutes before serving. This allows the casserole to set slightly.