Grease the inside of a 6-quart (or similar) slow cooker with cooking spray or butter. Line the bottom and up the sides with a long strip of parchment paper.

Mash the bananas in a large bowl until mostly smooth. Add granulated sugar, brown sugar, melted butter (or oil), eggs, and vanilla, then whisk until combined.

In a separate bowl, whisk together flour, baking soda, baking powder, salt, and cinnamon (if using).

Fold the dry ingredients into the wet mixture using a spatula until just combined. Do not overmix.

If desired, fold in nuts or chocolate chips.

Pour the batter into the prepared slow cooker and level the top. Sprinkle extra chips or chopped nuts on top if desired.

Place the lid on the slow cooker. Cook on HIGH for 2 to 3 hours or LOW for 3 to 4 hours. Start checking for doneness at the early time.

If you notice excess condensation on the lid during cooking, briefly lift and drain it.

When done, turn off the cooker and let the loaf rest inside for 10–15 minutes.

Use the parchment overhang to lift the bread onto a cooling rack. Let cool at least 20–30 minutes before slicing.