Grease the inside of the slow cooker insert lightly with butter or spray. Line 12 ramekins or mason jars with paper liners and place them inside the slow cooker. Add about 1 inch of water around the bottom of the ramekins to create a water bath for even cooking. Cover the slow cooker and set it to HIGH to preheat while you prepare the batter.

In a medium bowl, whisk together the flour, baking soda, salt, cinnamon, and nutmeg. Set aside.

In a large mixing bowl, whisk together the melted, cooled butter, brown sugar, and granulated sugar until well combined and slightly glossy.

Whisk in the eggs and vanilla extract to the wet mixture until just combined.

Fold in the mashed ripe bananas, leaving some small chunks for texture.

Pour the dry ingredients into the wet mixture. Gently fold with a spatula until no dry flour streaks remain. Do not overmix. If using, fold in chocolate chips or nuts now.

Divide the batter evenly among the prepared ramekins, filling each about two-thirds full. Sprinkle tops with a pinch of brown sugar if desired. Cover the slow cooker and cook on HIGH for 2.5 to 3 hours, or until a toothpick inserted into the center comes out clean. Note: Cooking times may vary by crockpot model.

Carefully remove the ramekins from the slow cooker. Allow muffins to cool in the ramekins for 10 minutes, then transfer to a wire rack to cool completely.