Preheat the oven to 150˚C (300˚F/gas 2).
Pound the mastic with a teaspoon of the icing sugar in a pestle and mortar until well ground. Set aside.
Cream the remaining icing sugar and butter in a large bowl with an electric beater until very pale and creamy, around 5-7 minutes.
Add the flour and mastic and beat to incorporate.
Add the orange blossom water and knead by hand until the dough is cohesive but very slightly crumbly (you may need a little more orange blossom water if the dough is too dry).
Roll into a log, wrap in cling film (plastic wrap) and refrigerate for 15 minutes. If not baking immediately, you can store it for longer, but when ready to bake, rest at room temperature until less stiff to handle.
Divide the dough into 4 pieces. Cover the rest while working with one piece.
Roll into a 1 centimeter (1/2 inch) thick rope, then cut it into 6-7 equal pieces.
Roll each piece into a ball shape, then press lightly with your thumb to make an indentation. Alternatively, coil each piece into a ring shape.
Place on a lined baking sheet.
Top each piece with a pistachio half. Repeat with the other pieces of dough.
Bake for 12-14 minutes, making sure they don't take on any colour.
Cool completely before moving as they will be fragile.
Store in an airtight container for up to 2 weeks.