Grease the interior of a 6-quart slow cooker generously with butter or cooking spray and line the bottom with a strip of parchment paper, leaving overhang on two sides for easy removal.

In a large mixing bowl, mash the ripe bananas thoroughly until few lumps remain. Whisk in the melted butter, granulated sugar, and brown sugar until completely smooth.

Beat in the eggs one at a time, followed by the vanilla extract and sour cream. Whisk until the mixture is glossy and well-combined.

In a separate medium bowl, whisk together the flour, cocoa powder, baking soda, and salt.

Gradually add the dry ingredients to the wet mixture. Stir gently until just combined to avoid overmixing.

If using, gently fold in the chocolate chips or chopped walnuts.

Pour the batter into the prepared slow cooker and spread evenly with a spatula. Cover and cook on HIGH for 2 hours, or on LOW for 4 hours, until the edges are set but the center is still slightly moist and fudgy.

Turn off the slow cooker and let the brownies cool completely in the slow cooker on a wire rack for at least 1 hour. Use the parchment overhang to lift them out and slice into 9 bars.