Preheat the oven to 150C (300C) and line the bottom of a square baking tin (21-22cm, 8-8.5 inches) with baking paper.
Put the butter and sugar into a small saucepan and put it on a medium heat. Keep stirring continuously until the sugar melts and comes together with the butter. Turn off the heat and add the vanilla, then mix together. Pour the caramel into the prepared cake tin and spread the caramel evenly in the tin.
Slice the bananas lengthways and place the banana on top of the caramel, ensuring the bananas fill up the space all over bottom of the tin. Set aside.
Prepare the dry ingredients. Sift the flour and baking soda into large mixing bowl. Add sugar into the same bowl and whisk the ingredients together to incorporate. Make a well in the centre of the flour mixture.
Add the beaten egg, mashed bananas and oil(wet ingredients) into the centre of the well(dry ingredients). Gently fold the ingredients until you see no clumps of flour, ensuring you scrape the sides and bottom of the mixing bowl. Place the banana cake mixture into the cake tin, on top of the caramel and banana.
Bake for 40-45 minutes or until a skewer comes out clean. Let the cake cool in the tin for around 10-15, then place a large plate on top the cake tin and flip the cake upside-down. Remove the baking paper and let cool. Serve warm or cold. Keep in an airtight container for up to 2 days at room temperature or in the fridge for up to 5 days.
* Percent Daily Values are based on a 2,000 calorie diet. Your daily value may be higher or lower depending on your calorie needs.