Lightly grease the inside of a 4- to 6-quart slow cooker with a bit of butter or neutral oil.
Rinse the raw white rice under cool running water until the water runs mostly clear. Drain well.
Spread the drained raw rice evenly across the bottom of the slow cooker insert.
In a large measuring cup or bowl, whisk together the whole milk, granulated sugar, vanilla extract, and ground cinnamon until the sugar is mostly dissolved.
Pour the milk mixture evenly over the raw rice in the slow cooker, ensuring all grains are submerged.
Cover the slow cooker with the lid and cook on LOW for 2 ½ to 3 ½ hours.
About halfway through the cooking time, quickly stir the mixture to prevent sticking.
Once the rice is soft and the pudding is creamy, turn off the slow cooker and let it sit covered for 15 to 20 minutes before serving.
Serve warm, sprinkled with extra ground cinnamon or chilled after refrigerating.