Coat the bowl of a 4-quart slow cooker with cooking spray.
In a large bowl, combine the eggs, butter, both sugars and vanilla until they are well-combined.
Add the flour, baking powder, baking soda and salt, and mix well, until the batter is smooth.
Fold in the mashed bananas and chopped walnuts. Stir until they are just combined.
Pour the mixture into the slow cooker and cover the top with a few paper towels. Place the lid on top of the towels and close tightly.
Cook on high for 4 hours until a toothpick inserted in the center comes out clean.
Once the slow cooker has cooled to the touch, run a butter knife around the edges to loosen the bread. Turn the bowl upside down on a platter to remove the bread.
Allow the bread to cool completely before slicing.