Preheat oven to 180 degrees & grease and line the base and sides of a 20cm square cake pan with baking paper
Place the butter and white chocolate into your Thermomix bowl and cook for 3 minutes, 60 degrees, speed 2 or until the chocolate has just melted.
Add the bananas, vanilla extract and egg and mix for 10 seconds, speed 5 to combine.
Scrape down the sides of the Thermomix bowl and add the flour, baking powder, caster sugar and mix for 10 seconds, speed 4.
Scrape down the sides of the Thermomix bowl and add the walnuts and chocolate chips and mix for 6 seconds, speed 5.
Pour batter into the pan and bake for 35 - 40 minutes or until golden, cool completely in pan
Once cooled, carefully remove from the baking tin and cut into squares.