Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish with butter or nonstick cooking spray.

Place the cubed sweet potatoes in a large stockpot and cover them with cold water. Bring to a boil over medium-high heat and cook for 15 minutes until fork-tender.

Drain the sweet potatoes thoroughly and return them to the pot or a large mixing bowl. Mash until smooth.

Add 0.5 cup melted butter, 0.5 cup granulated sugar, 0.25 cup brown sugar, 0.5 cup whole milk, 2 beaten eggs, 1 tsp vanilla extract, 0.5 tsp ground cinnamon, 0.25 tsp ground nutmeg, and 0.25 tsp salt to the sweet potatoes.

Beat or whisk the sweet potato mixture until smooth and creamy, then spread evenly into the prepared 9x13-inch baking dish.

In a medium mixing bowl, combine 1 cup packed brown sugar, 0.5 cup all-purpose flour, 1 cup chopped pecans, and the remaining 0.25 tsp salt.

Pour 0.5 cup melted butter into the pecan mixture and stir with a fork until crumbly streusel forms.

Scatter the pecan topping evenly over the sweet potato layer in the baking dish.

Bake at 350°F for 35 to 40 minutes, until the topping is golden brown and the casserole is set.

Let the casserole cool for 10 minutes before serving warm.