Preheat oven to 160 degrees Celsius (fan-forced).
Grease and line a rectangular loaf tin (approx 23cm x 13cm) and set aside until needed.
Place the plain flour, cocoa powder, salt and bi-carb soda into the Thermomix bowl. Sift 1 second, Turbo, 5-6 times then set aside in a separate bowl.
Place the butter and bananas into the Thermomix bowl and mix for 2 minutes, 100 degrees, Speed 2.5. Scrape down the sides of the bowl, before mixing for 10 seconds, Speed 5.
Add the brown sugar, egg and vanilla extract and whisk for 20 seconds, Speed 6.
Add the flour mixture and the chocolate melts and mix for 10 seconds, Speed 5. Scrape down the sides and mix for a further 5 seconds, Speed 5.
Pour mixture into the prepared loaf tin. Sprinkle the chocolate chips over the top.
Cook for 55 minutes or until a skewer inserted into the middle comes out with just a few crumbs on it (don't overcook though - you want a moist, fudgy loaf).
Allow to cool in the tin for 20 minutes and then transfer to a wire rack to cool completely.