Grease the bottom and sides of your slow cooker with butter or non-stick spray. Line with parchment paper if desired for easy removal. This prevents sticking in the moist crockpot environment.

In a large mixing bowl, mash the very ripe bananas until mostly smooth, leaving a few small chunks for texture.

Add the melted butter and neutral oil to the mashed bananas, stirring until fully combined. This combination is the simple secret for the best moist and tender texture.

Whisk in the granulated sugar, brown sugar, eggs, sour cream (or Greek yogurt), and vanilla extract. Whisk vigorously for about 60 seconds until the mixture is pale, slightly foamy, and the sugars are dissolving. This step emulsifies the wet ingredients for a velvety result.

In a separate medium bowl, whisk together the all-purpose flour, baking soda, salt, and cinnamon (if using). This ensures even dispersion of the leavening agent.

Gently fold the dry ingredients into the wet ingredients using a rubber spatula, just until no streaks of flour remain. Do not overmix to avoid a dense loaf. If there are chunks of banana, that's fine.

Pour the batter into the prepared slow cooker. Smooth the top with the spatula. If desired, sprinkle 1 tablespoon of granulated sugar on top for a sugar crust effect. Cover and cook on LOW for 3-4 hours or until a toothpick inserted into the center comes out clean or with a few moist crumbs. Do not remove the lid during cooking to maintain steady heat.

Once done, remove the insert from the slow cooker and let it cool in the insert for 10 minutes. Then carefully lift out using the parchment paper and transfer to a wire rack to cool completely. This prevents steaming and gummy texture.