Scrub sweet potatoes and pat dry. Prick each a few times with a fork. Bake sweet potatoes at 400F 45-50 minutes or until very fork-tender. Remove from oven and let cool 5 minutes.
Cut the top off of each sweet potato. Carefully scoop the filling out of the skin, being careful not to scoop too far down and break the bottom skin. Place sweet filling in a large bowl and empty potato skins in a greased baking pan.
Mash sweet potatoes thoroughly with butter, milk, sugar, vanilla, and cinnamon until smooth and no chunks remain (alternatively, you can use an electric mixer)
Fill sweet potato skins with sweet potato filling. Set aside for a moment to prepare the streusel.
Make the streusel: In a small bowl, toss all ingredients with a fork until butter is evenly incorporated and mixture is crumbly.
Sprinkle streusel evenly over filled sweet potatoes. Bake sweet potatoes at 375F an additional 15 minutes or until topping is golden-brown. Enjoy sweet potatoes piping-hot from the oven!