Preheat your oven to 350°F (175°C). Lightly grease a 9x13-inch baking dish.
In a large bowl, mix together the mashed sweet potatoes, granulated sugar, beaten eggs, melted butter, vanilla extract, and milk until smooth.
Spread the sweet potato mixture evenly in the prepared baking dish.
In a separate bowl, combine brown sugar, flour, chopped pecans, and melted butter to make the streusel topping.
Sprinkle the streusel evenly over the sweet potato mixture.
Bake for 30 minutes in the preheated oven until the topping is golden brown and crispy.
Remove from the oven and let it cool slightly before serving.