Grease the inside of the slow cooker with butter or oil to prevent sticking. Line the bottom with parchment paper if desired for easy removal.

In a large mixing bowl, mash the ripe bananas until they are mostly smooth with a few small lumps for a rustic texture.

Pour the melted butter or oil over the mashed bananas and stir until combined. Add the granulated sugar, brown sugar, egg, and vanilla extract. Mix vigorously for about 30 seconds until the mixture is uniform and sugars start to dissolve.

Sprinkle the flour, baking soda, salt, and cinnamon (if using) directly into the wet mixture.

Using a wooden spoon or spatula, fold the dry ingredients into the wet mixture. Mix only until no streaks of dry flour remain. The batter will be lumpy, which is perfect—avoid overmixing to keep it moist.

Pour the batter into the prepared slow cooker. Smooth the top with a spatula. Cover and cook on HIGH for 2.5 to 3 hours, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.

Do not remove the lid during cooking to ensure even heat. The loaf is done when the edges are set and pulling away slightly from the sides.

Turn off the slow cooker and let the loaf cool uncovered for 10-15 minutes inside the cooker. Then, carefully remove using the parchment paper and transfer to a wire rack to cool completely for at least another 15 minutes before slicing.