Melt the butter in a large pan over a low heat. Now add the brown sugar, milk and Marmite and stir until the sugar dissolves. Once the sugar has dissolved, allow it to start to bubble and boil for one minute. Place into a large bowl, add half the icing sugar and whisk thoroughly to combine. Leave to cool, mixing occasionally throughout the cooling process.
When fully cool, mix through the remaining icing sugar using an electric whisk until the mixture is pale and thick. If it is still runny add more until you have a thick spreadable consistency.