Preheat your oven to 350°. Pour the butter into a small baking dish (roughly 10 x 7 x 2.5 or 1-1/2 quarts), tilt the baking dish to coat the bottom and up on the sides a little.
In a medium bowl combine the milk and sugar and whisk to combine. Then add in the rice, raisins (if using), vanilla and cinnamon. Pour the mixture into your buttered baking dish and slide it onto the middle rack of your preheated oven and bake for 2 hours.
Serve as is or with a dusting of powdered sugar and more cinnamon if desired.