Grease the inside of your slow cooker or line it with a foil liner shaped like a loaf pan for easy removal.

In a large mixing bowl, whisk together the flour, baking soda, salt, and cinnamon (if using). Set aside.

In a separate bowl, combine the melted and cooled butter, brown sugar, and granulated sugar. Whisk until creamy and no lumps remain.

Beat in the eggs one at a time, then add the vanilla extract. Scrape down the sides as needed.

Stir in the mayonnaise. The mixture might look odd, but mix until mostly smooth and pale yellow.

Gently fold in the mashed bananas just until combined. Pour the wet mixture into the dry ingredients and stir carefully until just combined, with some flour streaks remaining.

If using, fold in the walnuts, pecans, or chocolate chips.

Pour the batter into the prepared slow cooker. Sprinkle a little sugar or cinnamon-sugar on top if desired. Cover and cook on LOW for 4 hours or until a toothpick inserted in the center comes out with only a few moist crumbs.

Once done, turn off the slow cooker and let the loaf cool uncovered in the crockpot for 10 minutes. Then carefully lift it out and cool completely on a wire rack before slicing.