Orange essence is available from speciality shops, but orange-flower water can also be used. This quantity of pastry is sufficient for six 12 cm tart shells or a 30 cm one.
Chill the bowl and blade of a food processor in the refrigerator.
Blend the butter, icing sugar and orange zest together to make a smooth paste, then add the egg and orange essence. Add the flour and pulse just until the dough comes together. Remove the dough from the bowl, then wrap it in plastic film and refrigerate for 2 hours.