Preheat oven to 170 degrees celsius (fan-forced). Grease and line an 18 X 28cm baking tray with baking paper and set aside.
Melt the butter in the Thermomix bowl on 80 degrees, Speed 2, 3 minutes (or until completely melted). Allow to cool slightly.
Add the eggs, plain flour, salt, baking powder, cocoa powder, brown sugar and caster sugar and mix on Speed 4, 5 seconds. Scrape down the sides of the bowl and repeat for a further 5 seconds or until completely combined.
Add the walnut, milk chocolate chips and white chocolate chips and mix on Reverse, Speed 3, 8 seconds.
Pour the mixture into the prepared tin and bake for 25-30 minutes or until crunchy on top and cooked through when tested with a skewer. Allow to cool in the pan before cutting into slices.