Preheat your oven to 350°F and grease a 9x13-inch baking dish with butter or non-stick cooking spray.
Place the peeled sweet potato chunks in a large pot and cover with cold water. Bring to a boil over high heat, then reduce to a simmer and cook for 15-20 minutes until the potatoes are fork-tender.
Drain the sweet potatoes completely and transfer them to a large mixing bowl. Mash them with a potato masher or electric mixer until smooth and free of lumps.
Add the 1 cup of brown sugar, 1/2 cup of softened butter, eggs, milk, vanilla extract, cinnamon, and salt to the mashed sweet potatoes. Beat with an electric mixer on medium speed for 2-3 minutes until the mixture is light and fully combined.
Spread the sweet potato mixture evenly into the prepared baking dish using a spatula.
In a small bowl, stir together the chopped pecans, flour, 1/4 cup of melted butter, and 1/4 cup of brown sugar until the mixture resembles coarse crumbs.
Sprinkle this topping evenly over the sweet potato layer. Bake for 30-35 minutes, or until the edges are bubbly and the pecan topping is golden brown and crisp.
Remove the casserole from the oven and let it rest on a wire rack for at least 10 minutes before serving to allow it to set.