Spray the bottom of a 1 1/2-quart souffle dish with cooking spray. Line the bottom of the souffle dish with parchment paper and give a quick spray with cooking spray; set aside.
Make a foil ring 1-inch thick for the souffle dish to sit on inside a 5-quart slow cooker.
In a medium bowl, combine the bananas with the hazelnut liqueur. Add the muffin mix, evaporated milk, oil, vanilla extract, cinnamon, chocolate chips and egg. Stir together until just blended.
Transfer to the prepared souffle dish. Place the dish on top of the foil ring in the slow cooker. Lay 8 paper towels over top of the slow cooker bowl and secure with the lid. (This helps to trap steam.) Cook on LOW setting for 4 to 5 hours.
Remove the cake and allow to cool completely.
Cut the cake in half horizontally. Stir the rum extract into the cream cheese frosting and frost in between the layers and outside of the cake with the mixture. Garnish the sides of cake with chopped walnuts. Slice and serve.