Grease the insert of your slow cooker with butter or non-stick spray. For easy removal, line it with parchment paper, leaving an overhang on the sides. Turn the slow cooker to LOW setting to preheat.

In a medium bowl, whisk together the flour, baking soda, salt, and cinnamon. Set aside.

In a large bowl, whisk the melted butter with the granulated sugar and brown sugar until smooth and combined.

Add the eggs one at a time, whisking well after each. Stir in the vanilla extract, mashed bananas, and sour cream until fully emulsified.

Pour the dry ingredients into the wet mixture. Using a rubber spatula, fold gently until just combined, with a few streaks of flour remaining. If adding walnuts, pecans, or chocolate chips, fold them in now.

Pour the batter into the prepared slow cooker insert. Smooth the top. Cover and cook on LOW for 3 to 4 hours, or until a toothpick inserted in the center comes out clean or with a few moist crumbs. Avoid opening the lid during cooking to prevent heat loss.

Turn off the slow cooker and let the bread cool for 10 minutes with the lid on. Then, remove the insert, use the parchment to lift the bread out, and transfer to a wire rack to cool completely before slicing.