Preheat the oven to 160°C (325°F). Grease a cake tin with butter and line it with baking paper.
To make the miso caramel, heat the sugar in a saucepan until it turns into a light golden brown caramel. Remove it from the heat and add the butter and miso, being careful as it may spit. Pour the caramel into the prepared cake tin.
Arrange the halved bananas on top of the caramel in the tin.
In a mixing bowl, cream together the softened butter, dark brown sugar, and vanilla extract for 8-10 minutes until the mixture is pale and creamy. Scrape down the sides of the bowl.
Gradually add the eggs and beat well to combine. Add the mashed banana, plain flour, baking powder, bicarbonate of soda, cinnamon, and golden syrup. Mix everything together until well combined.
Spoon the cake mixture over the bananas and caramel in the tin.
Bake for 60-65 minutes or until a skewer inserted into the center comes out clean.
Leave the cake to cool in the tin for 20 minutes before turning it out onto a wire rack to cool completely.
Once cool, slice and serve the cake with a pat of extra butter.