Place the peeled and cubed sweet potatoes in a large pot and cover with cold water. Bring to a boil over medium-high heat, then reduce heat to medium and simmer until the potatoes are very tender when pierced with a fork, about 15-20 minutes. Drain well.

Return the drained sweet potatoes to the pot or transfer to a large mixing bowl. Mash them thoroughly with a potato masher until smooth, or use an electric mixer on low speed.

To the mashed sweet potatoes, add the melted butter, granulated sugar, 1/4 cup light brown sugar, milk, beaten eggs, vanilla extract, cinnamon, nutmeg, and salt. Mix until well combined and smooth.

Preheat your oven to 350°F (175°C). Lightly grease a 9x13 inch baking dish.

Pour the sweet potato mixture into the prepared baking dish and spread evenly.

In a separate medium mixing bowl, combine the all-purpose flour, 1/2 cup light brown sugar, and chopped pecans. Add the cold, cubed butter. Using your fingertips or a pastry blender, cut the butter into the dry ingredients until the mixture resembles coarse crumbs.

Sprinkle the pecan streusel evenly over the sweet potato mixture. Bake for 30-35 minutes, or until the sweet potato mixture is set and the streusel topping is golden brown and bubbly.

Remove the casserole from the oven and let it rest for 5-10 minutes before serving warm.