Melt the butter in a small saucepan and stir in the brown sugar and milk, then bring to the boil. Cook, stirring for about 3 mins, then cool. Add enough sifted icing sugar to give a spreading consistency, stir until smooth. Spread on top of cold cake or cupcakes.
Recipe by Coloured Sugar at https://www.colouredsugar.com/caramel-frosting/