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.
Mash the bananas in a large bowl.
Add the brown sugar, melted butter, egg and vanilla. Whisk well.
Sift the plain flour, cocoa, salt and bi-carb soda over the top. Stir until just combined.
Add 1/2 cup dark chocolate melts and stir through.
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).