Line the bottom and sides of your 6-quart or larger slow cooker with parchment paper, leaving an overhang on the sides to use as handles later. You can also grease it generously with butter or non-stick spray, but parchment paper helps prevent sticking and makes removal easier.

In a large mixing bowl, mash the ripe bananas until mostly smooth, with a few small lumps remaining if desired. Add the melted butter, granulated sugar, brown sugar, vanilla extract, eggs, and buttermilk. Whisk until well combined.

In a separate medium mixing bowl, whisk together the all-purpose flour, baking soda, salt, and ground cinnamon (if using).

Gradually add the dry ingredients to the wet ingredients, mixing with a rubber spatula until just combined. Be careful not to overmix. If using, fold in the chopped nuts or chocolate chips.

Pour the batter into the prepared slow cooker, spreading it evenly. Place a few layers of paper towels directly over the top of the slow cooker insert, then cover with the lid. The paper towels will absorb condensation and prevent it from dripping onto the bread, which helps create a nicer crust.

Cook on HIGH for 2.5 to 3 hours, or on LOW for 3.5 to 4 hours. The cooking time can vary based on your slow cooker's model and heat. The banana bread is done when a wooden skewer or toothpick inserted into the center comes out clean or with moist crumbs, but no wet batter.

Once cooked, turn off the slow cooker and remove the lid and paper towels. Carefully lift the banana bread out of the slow cooker using the parchment paper handles. Transfer it to a wire rack to cool completely before slicing and serving.