Place the roughly chopped carrot and parsnip into a food processor and whizz until finely choppedAdd the nuts and process further until tiny piecesAdd the banana and flour and process againFinally add all the other cake ingredients and process for about 3-4 minutes until the mixture is smoothWhilst the mixture is being made, grease a 20cm square tin and pre-heat your oven to 200 degrees, 180 degree fan.Pour the mixture into the prepared tin and bake in the pre-heated oven for 30 - 35 minutes until springy to the touch and cooked through. Cool on a wire rack before toppingWhile the cake is cooking prepare the topping if using.Mix all the ingredients together and place in the fridge until ready to spread on top of the cold cake