• Preheat the oven to 180°c

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.
• Weigh the flour, baking powder and salt into the TM bowl and sift with the MC in place on speed 3/5 seconds then remove and set aside
• Weigh the banana into the TM bowl and chop on speed 3/6 sec and remove
• Without washing the TM bowl weigh in the butter and sugar and beat on speed 5/10 seconds.
• Add the butterfly into the TM bowl. Add the eggs, sour cream, vanilla and mix on speed 2/10 seconds
• Return the flour mix, banana and walnuts and mix again on speed 3/10 seconds
• Pour and 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, you can begin to make the frosting.
• Weigh the cold cubes of butter into the TM bowl, beat for 30 seconds/speed 5, then weigh in the cream cheese and vanilla. Beat again for around 10 seconds/ speed 5 until light and creamy. This will depend on how cold your ingredients are, but check around 10 seconds, then scrape the base and sides of your bowl and go again if it's still not soft and fluffy.
• Weigh about 3/4 of the icing and beat on speed 3 with the MC in place. You'll need to keep an eye on it; it should take around 15 seconds, with a scrape down in between. Add more icing sugar, beating as you go until you reach the desired texture.