Heat oven to 350 degrees F.

In a large bowl, stir together mashed sweet potatoes, sugar, melted butter, milk, vanilla, and eggs until well combined. Pour and spread evenly into a casserole dish.

In a medium bowl, combine chopped pecans, brown sugar, flour, and softened butter until mixture forms a coarse crumb. Sprinkle streusel evenly over the top of the casserole.

Bake for 40 minutes until casserole is baked through and streusel topping is golden brown and bubbling.