Line your slow cooker with a slow cooker liner or parchment paper to prevent sticking.

In a large mixing bowl, thoroughly mash the ripe bananas until mostly smooth with a few lumps for texture. Stir in the melted butter until combined. Add the sour cream, eggs, and vanilla extract. Whisk gently until the mixture is smooth and emulsified.

In a separate medium bowl, whisk together the flour, sugar, baking soda, and salt until evenly distributed.

Pour the dry ingredients into the wet ingredients. Fold gently with a wooden spoon or spatula until just combined, avoiding over-mixing to keep it moist.

If using chocolate chips or nuts, fold them in gently now.

Pour the batter into the prepared slow cooker. Cover and cook on low for 3.5 to 4 hours or on high for 1.5 to 2 hours, until a skewer inserted in the center comes out clean or with a few moist crumbs.

Turn off the slow cooker, remove the lid, and let the loaf cool inside for 15 minutes. Then, using the liner or parchment, lift it out and transfer to a wire rack to cool completely before slicing.