Preheat oven to 180C/350F/Gas4
Line a 10 x 14" baking tray with sides at least 1" high
Mash the bananas to a pulp and stir in the vanilla extract
Sift together all the dry ingredients
Add the dry ingredients to the bananas a little at a time
Add the eggs one at a time beating as you go
Pour in the oil between the additions of dry ingredients (you want to add wet - dry- wet - dry)
Make sure everything is well combined and pour into your prepared baking tray
Bake for approximately 30 - 40 minutes, checking after 30 mins by inserting a skewer into the centre, if it comes out clean the cake is done.
Let it cool on a wire rack while you make the topping.
Place the butter (make sure it is softened but not melted) in a large bowl
Add the caramel and stir together
Start stirring in the icing sugar a little at at time. This is where a big bowl is important as it flies all over the place. Keep adding until you get a good paste consistancy that you like
When the cake is completely cool, spread topping all over and cut into squares.