3 egg whites, at room temperature
2 tbsp caster sugar (or in my case, homemade vanilla sugar)
200 g powdered sugar
110 g pistachios
green gel (or powder) food coloring

Combine powdered sugar and pistachios in your food processor, and grind until very fine. Sieve. Re-grind any lumps or big pieces of almonds. Mix carefully (they tend to separate a little bit when sieving, since the sugar falls through first.)

Beat egg whites and sugar until you have a thick, glossy meringue. Don't overbeat. Stir in the almond-sugar powder, and food coloring if you'd like, and fold together. Don't overmix this - most seem to say that you should use less than 50 strokes. (You can try it by dolloping a little batter on a baking sheet - if the peak falls down, it's ready. If it doesn't, try a few more strokes.)

Pipe small rounds on a baking sheet with baking paper. If you'd like, sprinkle some extra ground pistachios on half of them. Leave at room temperature for 30-60 minutes, to form a skin.

Bake at 150°C for 15-20 minutes. (I have a convection oven, and 15 minutes was perfect for me.) Let them cool completely before filling. Unused shells can be frozen, or kept in an air-tight container for a day or so.