Heat oven to 160C/140C fan/gas 3 and line a 12-hole muffin tin with cases. Put 85g pistachios into a food processor with about half the sugar, then whizz until very finely chopped. Tip into a large mixing bowl with the remaining sugar, butter, eggs, flour and milk and beat until smooth. Divide between cases, then bake on a low shelf for 22-25 mins until a skewer poked in comes out clean. Cool on a wire rack.
For the icing, mix just enough water into the icing sugar to get a very thick but still runny icing – just add 1 tbsp of water at a time. Stir in enough food colouring to give a pretty, pale green colour. If any cakes poke above the top of the cases, trim with a small knife, or scoop out with a teaspoon – being very careful not to release any of the case from the sides of the cake. Put a generous spoonful of icing on each cake and let it gently spread to cover. Chop the remaining pistachios and scatter these over with a pinch of edible glitter. Set, then serve.