In a large bowl add the cake mix, melted butter, mashed bananas, water, vanilla extract, and eggs. Whisk until combined, do not worry if there is a few lumps left of cake mix, that is normal. Add about 3/4 of the nuts and stir again. (Saving some nuts for topping the frosting)
Spray the slow cooker very well with non-stick spray. Add the batter to the slow cooker and smooth out to a flat layer.
Cover and cook on HIGH for 2.5 hours, do not open the lid during the cooking time.
When the cooking time is done. Remove the insert out of the heating unit. Set the ceramic on a cooling rack. Let cake cool completely, this may take a few hours. To speed up the process after the ceramic isn't piping hot, you can put it in the fridge to cool.
When the cake is cooled, start making the frosting. You can use a stand mixer, or a handheld electric mixer.
Blend together the cream cheese and butter until combined. Add the powdered sugar, vanilla and milk. Blend until smooth, scrape down the sides of the bowl and blend again.
Spread the frosting on the cooled cake. Sprinkle with reserved nuts. Slice and serve.
Nutritional values are approximate, please use your own calculations if you require a special diet. The values below are based on 1/8 of the entire recipe.