method
pre heat oven to 175°C
Put the icing sugar and pistachio into a blender and process until very fine, then tip into a mixing bowl. Whisk the egg whites until they are stiff and then slowly whisk in the sugar. Fold the meringue into the pistachio and icing sugar and then pipe onto sheets covered with silicon or baking parchment. Bake until crisped and very lightly coloured — about 15 minutes.
Working one baking sheet at a time, immediately put about a tablespoon of water onto each tray and roll it around. Allow to cool completely and turn them upside down on the tray.
for the pistachio butter
Process the icing sugar and nuts until very fine in a blender. Cream the butter and egg yolk, then add to the nut and icing sugar mix folding it together carefully.
to assemble
Pipe a small amount of the pistachio butter onto a macaroon and push another on top.