In a bowl, combine butter, eggs and sugar and mix well.
Add in baking powder, baking soda, salt and 1 cup flour and mix until combined. Add in the other cup of flour and mix.
Mash bananas well and then add to mixture. Add walnuts into the mixture.
Butter removable dish in the slow-cooker and pour mixture in.
Cook for 4 hours on low. The top should be golden and when you touch the cake it should have a bounce to it.