Sift the flour and baking powder into a bowl. In a separate bowl whisk together the sugar, yogurt, eggs, oil and banana. Then add to the flour to make a paste before finally mixing in the carrots and walnuts.
Grease a 20cm sponge tin and line with greaseproof paper. Add the cake mix. Bake at 180°C for about 45 minutes to an hour until cooked.
To make the topping place the cheese, butter, icing sugar and vanilla in a food processor for 20 seconds until smooth and spread onto the cake when it has cooled. Decorate with more walnuts and refrigerate for an hour before serving.