Premake either the Notella or Mocha Walnut Spread and brown your butter as well, so it can cool slightly prior to adding to the mix.
Preheat oven to 175°C and grease a 23 x 13cm loaf tin. Set aside.
Place brown butter, bananas, sugar, eggs, whey, vanilla bean paste, salt and bicarb soda into the Thermomix bowl and blend 20 sec/speed 7 If your bananas are frozen, make sure they have been completely blended into the mixture.
Add flour and blend 10 sec/speed 7. Scrape down sides of bowl and repeat.
Scrape 1/2 the mix into the prepared tin and dollop some of the Notella on top. Finish off pouring the batter on top of that and then add the remaining Notella. Swirl the mixture together using a palate knife or offset spatula. Don't overwork it or you will lose the marbled effect.
Sprinkle the chocolate callets on top if using. Bake 50-55 minutes until well risen and fragrant. You will not be able to use the normal skewer method to test doneness for this as the Notella or Mocha Walnut Butter will stay sticky when hot.
Remove from oven and cool on a wire rack before removing from the tin. Cut and serve with more Notella or Mocha Walnut Butter. Cutting into thick slices and freezing individually is the best way to store this...plus you may forget about it if it is out of sight so it will last a little longer. (But probably not!)