Grease the insert of a 5-6 quart slow cooker with butter or non-stick spray, or line it with parchment paper for easy removal. Plug in the slow cooker to preheat on low if possible, but this step can be skipped if your model doesn't preheat.

In a large mixing bowl, thoroughly mash the very ripe bananas using a fork or potato masher until mostly smooth with a few small lumps for texture.

Add the melted butter, granulated sugar, and brown sugar to the mashed bananas. Stir until smooth and well combined. Whisk in the room temperature eggs one at a time, then add the vanilla extract and mix.

In a separate medium bowl, whisk together the flour, baking soda, and salt to distribute evenly. (Optional: Sprinkle these directly over the wet ingredients if preferring strict one-bowl method.)

Pour the dry mixture into the wet mixture. Stir gently just until the flour streaks disappear—overmixing leads to tough bread. Fold in optional mix-ins like walnuts or chocolate chips if using.

Pour the batter into the prepared slow cooker insert. Cover and cook on HIGH for 3 to 4 hours or on LOW for 6 to 8 hours. Start checking doneness at the 3-hour mark on high (or 6-hour mark on low) by inserting a toothpick into the center—it should come out with a few moist crumbs, not wet batter. If the top browns too quickly, loosely tent with foil.

Turn off the slow cooker and carefully lift out the bread (insert should be cool enough to handle). Let it cool completely on a wire rack before slicing to maintain moisture and structure.