cordshoes The chocolate in Flakes is dehydrated during processing, so that it crumbles better in the mouth. When the Flake is directly heated without moisture e.g. in a microwave, the sugar burns and hardens, and the melting fat can't lubricate the chocolate particles to allow them to flow apart.
However, if you supply moisture, be it from a hot drink or sweat from your hand, it can melt.
