Preheat your oven to 180 degrees and line the base of a DEEP 20cm round cake tin.
Place the butter and caster sugar into your Thermomix bowl and mix for 20 seconds, speed 2.5 to cream. Scrape down the sides of your bowl and mix for a further 30 seconds, speed 3 or until light and creamy.
Add the bananas and eggs and mix for 10 seconds, speed 5 to combine.
Scrape down the sides of your Thermomix bowl and add the self raising flour, bicarb soda and cinnamon and mix for 10 seconds, speed 5.
Scrape down the sides of your Thermomix bowl and mix for a further 7 seconds speed 5 to combine..
Spoon the cake mixture into a lined and deep 20cm cake pan (or a 22cm ring tin) and bake for 50 minutes (if cooking in a round tin) or 35 minutes if baking in a ring tin or until the cake is cooked when tested with a skewer