Grease the slow cooker insert generously with butter or non-stick spray, or line with parchment paper for easy removal. Set to LOW setting.

In a large mixing bowl, mash the ripe bananas until mostly smooth, leaving a few small lumps for texture.

Stir in the melted butter, granulated sugar, brown sugar, eggs, and vanilla extract. Whisk until well combined but not smooth.

Gently fold in the sour cream until just combined and no streaks remain. This is the key to unbeatable moisture.

In a separate bowl, whisk together the flour, baking soda, salt, and cinnamon.

Add the dry ingredients to the wet mixture. Stir with a spatula or spoon until just combined; do not overmix to avoid a dense loaf.

If using, fold in the walnuts or chocolate chips.

Pour the batter into the prepared slow cooker. Cover and cook on LOW for 4 hours, or until a toothpick inserted in the center comes out clean or with moist crumbs (not wet batter). The loaf will rise and set like a traditional baked bread.

Turn off the slow cooker and remove the insert. Let the bread cool in the insert for 10-15 minutes before carefully lifting out (using parchment if lined) and transferring to a wire rack to cool completely.