Step 1: Preheat your oven to 400°F (200°C). Pierce the sweet potatoes several times with a fork. Place them on a baking sheet and roast for 45-60 minutes, or until very tender. The skin should be easily peelable. Let cool slightly.
Step 2: Once cool enough to handle, peel the sweet potatoes. Place the warm, peeled sweet potatoes in a large mixing bowl. Add the softened unsalted butter, granulated sugar, 1/4 cup brown sugar, milk, vanilla extract, cinnamon, and 1/4 tsp salt. Mash with a potato masher or use an electric mixer on medium speed until smooth and creamy.
Step 3: Spread the sweet potato mixture evenly into a greased 9x13-inch baking dish.
Step 4: To make the streusel topping, in a medium bowl, combine the all-purpose flour and 1/2 cup brown sugar. Add the cold unsalted butter (cut into small pieces). Use your fingertips or a pastry blender to cut the butter into the flour mixture until it resembles coarse crumbs.
Step 5: Stir in the chopped, toasted pecans. Spread the streusel mixture evenly over the sweet potato base in the baking dish.
Step 6: Bake in the preheated 375°F (190°C) oven for 25-35 minutes, or until the casserole is heated through and the streusel topping is golden brown and bubbly. If the topping browns too quickly, you can loosely tent it with aluminum foil.
Step 7: Remove from the oven and let it rest for 10-15 minutes before serving. Serve warm.