Dust the work surface very lightly with icing sugar and knead the Marzipan to soften it. Flatten slightly then add 15g (1tbsp) ground pistachios. Fold over the edges of the Marzipan to enclose the nuts and continue kneading to distribute the nuts. Repeat the folding to use up all the nuts, adding a few drops of Lime Green Gel Food Colour, if using, with the last batch to give a light green shade.
Form the Marzipan into an even length about 30cm (12") long. Square off the edges and then cut into 20 square chunks.
Break the Milk and Extra Dark Chocolate into a heatproof bowl. Put the bowl over a saucepan of barely simmering water and leave to melt. Remove from the water and leave aside for 10 minutes to cool.
Line a board with baking parchment. Working on one square of marzipan at a time, drop it into the chocolate and gently turn it over using a fork to coat it completely. Carefully lift out the square on the prongs of the fork and gently tap the fork several times on the side of the bowl to remove the excess chocolate and give a smooth finish. Put the square on the lined board and put a piece of pistachio nut on top. Repeat this process to cover the remaining pieces of Marzipan.
Put the chocolates in a cool place to set. If the temperature is warm, cool the chocolates in the fridge for about 15 minutes.