Whisk the egg whites and salt until stiff and dry, then whisk in the sugar a little at a time until you reach a marshmallowy consistency.
Add lemon zest, vanilla extract, almond extract and ground almonds. Mix to a hard paste.
Shape into small diamonds - dust hands and surfaces with icing sugar to stop the mixture sticking (we ommited this).
Lay on lined baking sheets and leave to dry out overnight.
Heat the oven to 140 °C and cook the biscuits for 25-30 mins - they should be pale and slightly cracked.
When cool, dust with icing sugar and serve. (we ommited this).