Preheat oven to 180°C. Grease and line the bottom and sides of a 20cm cake tin with baking paper.
In a food processor or blender, blend oil, sugar, eggs, banana and yoghurt together until smooth.
Into a large mixing bowl, sift flour, baking soda and baking powder. Add banana mixture and stir until well combined but take care not to over-mix.
Spoon batter into lined cake tin, gently smooth out the top and bake for about 50 minutes or until cake springs back when lightly touched. Leave to cool in the tin for 10 minutes before turning out. When cake is cold, use a serrated knife to cut it in half horizontally.
Make icing while cake is cooking. Beat butter, cream cheese, vanilla and brown sugar together until creamy. Sift over icing sugar and beat in until well combined. Chill icing in the fridge for at least 20 minutes before using (this will firm it up, making it easier to use).
To assemble cake, place one half on a serving plate or cake stand. Spread with chilled whipped cream. Arrange sliced banana over the top and dollop with half of the caramel condensed milk.
Place the other half of the cake on top. Then ice top and sides of cake with the chilled icing – you don’t have to be too perfect; the slightly rustic look is cool! Add more dollops of caramel condensed milk and drizzle with about ¼ cup cold salted caramel sauce. Garnish with sliced banana, chocolate shards and walnuts.