Give the potatoes some time to cool, then cut them in half and scoop out the potato from the skins. Be sure to leave a nice buffer for yourself between your scooping and the skin of the potato. Add the scooped potato into a large bowl. Use your hand masher to mash your sweet potato slightly. Add butter, milk, brown sugar, vanilla, and cinnamon. Stir well to combine. Fill the sweet potato skins back in with the sweet potato mixture.