Preheat oven to 160 degrees celsius fan-forced (320 degrees fahrenheit). Grease and line a non-stick loaf tin (approximately 21cm x 11cm or 8 inch x 4.5 inch) and set aside.
Place the softened butter and brown sugar into the Thermomix bowl. Mix for 20 seconds, Speed 4. Scrape down the sides of the bowl and repeat for a further 20 seconds.
Add the bananas and mix for 5 seconds, Speed 5. Scrape down the sides of the bowl.
Add the eggs and mix for 5 seconds, Speed 5.
Add the plain flour, bi-carbonate soda, salt and vanilla extract and mix for 5 seconds, Speed 5. Scrape down the sides of the bowl and repeat for a further 5 seconds.
Pour the mixture into the prepared tin. Sprinkle a little raw sugar over the top (optional).
Cook for 1 hour or until a skewer inserted into the middle comes out clean or with a few crumbs.