Lightly grease the inside of the slow cooker with butter or cooking spray.
Arrange the sliced bananas evenly across the bottom of the slow cooker.
Sprinkle the brown sugar evenly over the bananas.
Dot the top with butter pieces, drizzle with vanilla extract, and sprinkle cinnamon if using.
Cover and cook on LOW for 2-3 hours until the bananas are very soft and the sauce is thick and caramelized.
Serve warm on its own or spoon over ice cream, pancakes, waffles, oatmeal, or pound cake.
Notes
Use ripe bananas for best flavor. For a thicker sauce, cook uncovered during the last 20-30 minutes. Add nuts during the final 30 minutes for extra texture.