• Preheat the oven to 180°.

For the cake

• Toast the walnut pieces for ten minutes or until they become fragrant or crunch when you break them with your hands. Watch out they're hot when they come out of the oven.
• Next, in a bowl, cream together the butter and sugar until light and fluffy. Add the eggs, sour cream, Vanilla, and stir to combine. Then, add the mashed banana and combine.
• In a separate bowl, combine the flour, baking soda and salt and gradually add the dry ingredients to the creamed mixture. Stir in the toasted walnuts.
• Spread into a greased 12×5-inch baking pan. Bake until a toothpick comes out clean Remove from the oven and allow to cool completely in the pan. The surface will become quite golden brown but don’t worry; it’s all about the sugars caramelising.

For the topping

• While the traybake is cooling begin to make the frosting. Beat together the cream cheese, butter and Vanilla. Once combined, gradually beat in enough confectioners’ sugar to achieve desired consistency. Frost the bars and store them in the refrigerator until you are ready to serve.