Place the peppermints in a polythene bag and crush them with a rolling pin, breaking them into small pieces. Continue to roll and flatten the mints until they are ground to a powder. Line a shallow 18 cm (7 inch) baking tin with nonstick baking paper.
Put the plain dark chocolate and condensed milk in a heatproof bowl over a pan of gently simmering water. Leave until melted, stirring frequently. When melted, stir in the ground peppermints.
Beat the mixture until the ingredients are combined then turn it into the tin, spreading it into the corners. Level the surface and leave to cool. Chill in the refrigerator for at least 2 hours.
Lift the fudge out of the tin and peel off the paper. Melt the milk or white chocolate in a heatproof bowl over a pan of gently simmering water, then place in a paper piping bag. Snip off the tip and scribble lines of chocolate over the fudge. When the chocolate is set, cut the fudge into 2 cm (¾ inch) squares. It will keep for up to 2 weeks in the refrigerator.