Preheat oven to 175 degrees celsius (160 if using fan-forced). Grease a rectangular loaf tin (approx 23cm X 13cm) with butter and set aside.
Place the plain flour, cocoa and bi-carb soda into the Thermomix bowl. Sift by pressing Turbo 5-6 times then set aside in a seperate bowl.
Place the butter and bananas into the Thermomix bowl and mix on 100 degrees, Speed 2.5 for 2 minutes. Scrape down the sides of the bowl. Mix on Speed 5 for 10 seconds.
Add the brown sugar, egg, salt and vanilla and whisk on Speed 6, 20 seconds.
Add the flour mix and the chocolate melts and mix on Speed 5, 10 seconds. Scrape down the sides and mix for a further 5 seconds.
Pour mixture into the prepared loaf tin. Sprinkle over the choc chips.
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 for 20 minutes and then place onto a wire rack to cool completely.
Store in an airtight container for up to 4 days at room temperature (or freeze in an airtight container for up to 2 months).