Over low heat combine the milk and custard powder and whisk until very thick. Set aside to cool completely. (now is a good time to make your cupcakes). Beat juice, butter and cool custard until smooth. Gradually add in icing sugar until you get desire consistency. Please note, this makes heaps of icing! It easily does 24 with piped high icing or would do 48 spread icing no worries. Leave cakes uncovered for half an hour or so until icing gets it's crust.