Line a 4-6 quart slow cooker with parchment paper, leaving an overhang on two sides for easy removal. Lightly grease the parchment paper.

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

Add the eggs one at a time, beating well after each. Stir in the vanilla extract and mashed ripe bananas. If using Greek yogurt or sour cream, fold it in now.

In a separate bowl, whisk together the all-purpose flour, cocoa powder, baking soda, and salt until evenly distributed.

Gently fold the dry ingredients into the wet mixture using a spatula. Mix only until just combined, with a few flour streaks remaining to avoid over-mixing.

If using, fold in the dark chocolate chips or chopped nuts. Transfer the batter to the prepared slow cooker and spread it evenly.

Cover and cook on low for 2.5 to 3 hours, or until the center is set but still fudgy. The edges should be slightly cracked. Do not overcook for maximum Moisture.

Turn off the slow cooker and let the brownies cool completely inside with the lid ajar for about 1 hour. Use the parchment overhang to lift them out, then slice into squares.