Place the sifted powdered sugar, condensed milk, peppermint extract, and 2 tablespoons of water into a large mixing bowl.
Stir the mixture thoroughly until it comes together as a firm, stiff paste.
If the paste appears overly dry or crumbly, incorporate more water 1 teaspoon at a time until the mixture holds together.
If the mixture becomes too loose, blend in a little more powdered sugar until a firm working consistency is restored.
Lightly coat a clean work surface and rolling pin with powdered sugar to keep the paste from sticking.
Roll the peppermint mixture out evenly to approximately 0.5 cm, or about 1/4 inch, in thickness.
Dust a 4 cm, or roughly 1.5 inch, round cutter with powdered sugar, then press out 24 circles from the rolled peppermint paste.
Arrange the rounds on a tray lined with parchment paper and leave them to dry slightly before adding the chocolate decoration.
Place the dark chocolate in a heatproof bowl set over a pan of barely simmering water.
Allow the chocolate to melt gently, stirring from time to time, until smooth and glossy.
Remove the bowl from the heat once the chocolate has melted, then let it cool slightly.
Transfer the melted chocolate into a piping bag fitted with a small plain nozzle.
Pipe the chocolate over the peppermint rounds in a zig-zag design, decorating each piece evenly.
Move the decorated peppermint creams to the refrigerator for about 10 minutes, or until the chocolate has fully set.
Serve once set, or store the peppermint creams for later enjoyment.