Preheat the oven to 350°F (175°C).
In a large mixing bowl, combine the mashed sweet potatoes, granulated sugar, beaten eggs, melted butter, milk, and vanilla extract. Mix until well combined and smooth.
Transfer the sweet potato mixture to a greased 9x13-inch baking dish, spreading it evenly.
In a separate bowl, prepare the pecan streusel topping by combining the chopped pecans, brown sugar, flour, melted butter, and ground cinnamon. Stir until the mixture resembles coarse crumbs.
Sprinkle the pecan streusel topping evenly over the sweet potato mixture in the baking dish.
Carefully transfer the casserole to the preheated oven. Allow it to bake for approximately 30 to 35 minutes, or until the topping achieves a golden brown color and the sweet potato mixture sets perfectly.
Once the baking time is complete, carefully take out the casserole from the oven. Give it a few minutes to cool down before serving.