Melt the butter in a heavy-based or non-stick saucepan over a medium heat.
Add the saffron strands and sugar and cook stirring continuously for a couple of minutes until the sugar melts.
Add the milk and simmer, stirring for a minute until the mixture bubbles.
Tip in the milk powder and continue simmering, stirring for a couple of minutes until the mixture becomes stiff and and begins to leave the sides of the pan.
Put the mixture onto a sheet of greaseproof paper and leave it to stand for a minute, then use a wet palette knife or spatula to shape it into a 10cm square about 1cm thick. Sprinkle over the nutmeg.
Break the chocolate into small pieces and place in a heatproof bowl and set aside.
Add a little boiling water to a saucepan and heat until simmering. Suspend the bowl of chocolate over the simmering water (this is known as a bain marie) and allow the chocolate to melt. Chocolate needs to be heated carefully to prevent it from seizing through overcooking. Melt the chocolate just before you intend to use it. When the chocolate is almost completely melted, take it off the heat and spread it over the top of the spiced milk fudge base.
Place in the fridge and leave to set until firm but not completely solid. Cut into squares.