In a bowl mix together 250g plain flour with 2 tspn baking powder and 1/2 tspn bicarb. Mix in 250g caster sugar.
In another bowl beat together 150ml sunflower oil, 250g ripe bananas [ I used 2 large ones], 2 eggs, 50ml natural yoghurt and 1 tspn vanilla extract.
Pour the oil mixture into the flour and stir together till mixed.
Spoon the batter into the cake tin and bake for about 40 mins. Leave to cool in the tin for 10 mins, then turn onto a wire rack.
For the caramel topping:
Put 85g butter, 175ml double cream and 175g soft brown sugar in a pan over a medium heat and stir till the butter has melted. Turn up the heat and boil for 3-4 mins stirring occasionally - mixture looks like runny custard. Pour into a bowl and leave for 30 mins, then beat with an electric hand beater till thick.
When the cake is cool, spread the icing over the top.
I had some topping left over so put it in the fridge and used it with vanilla ice cream. It had firmed up, but was still delicious.