Insert a sharp knife into one end of the bananas to create a slit in the skin and insert the sprig of fresh rosemary. Heat a griddle pan until very hot.
Place the bananas with the skin still on onto the griddle pan and griddle for 3-4 minutes each side, or until the skin is dark brown and starting to split.
To make the toffee sauce, simply place the double cream, butter, sugar, treacle and golden syrup into a saucepan over a medium heat and bring to the boil. Stir well to ensure all the ingredients are combined and dissolved.
To make the ice cream, tip the frozen banana chunks into a food processor. Add the vanilla essence, sugar and 75ml/2½fl oz of the buttermilk. Turn on the processor and let it run for 20-30 seconds.
While the food processor is still running, gradually pour in the remaining buttermilk in a steady stream. Let the machine run until the mixture is smooth and creamy.
Serve each banana in a bowl as you would a jacket potato: cut open down the centre, with the skin on. Pour over a spoonful of the toffee sauce and place a scoop of ice cream on top.