Set your slow cooker to low and grease a loaf pan or line it with parchment paper for easy lifting. Mash the ripe bananas with a fork until mostly smooth.

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

In a large mixing bowl, beat the slightly cooled melted butter and brown sugar together until combined. Add the eggs one at a time, followed by the vanilla extract, beating until the mixture is light and fluffy.

Gently stir in the mashed bananas and Greek yogurt until barely combined.

Gradually add the dry ingredients to the wet ingredients. Mix only until just combined and no streaks of flour remain. Fold in optional add-ins if using.

Pour the batter into the prepared slow cooker. Cover and cook on low for 3-4 hours, or until a wooden skewer inserted into the center comes out clean or with minimal moist crumbs.

Allow the loaf to cool in the slow cooker for 10 minutes, then carefully transfer to a wire rack to cool completely.