1. Preheat the oven to 200oC.
2. Cover a baking tray with silicone paper.
3. Gently break up the egg whites with a fork.
4. Put the ground almonds into the food processor and process for about 15 secs until very fine. Add about 1/2 of an egg white and process for about 10 secs.
5. Add 1/2 the caster sugar and process for 10 secs.
6. Repeat with another 1/2 an egg white and the remaining sugar.
7. Then add a further 1/2 egg white.
8. The mixture should be quite soft and it should be possible to form it into about 12 balls the size of a large walnut. If it is too stiff add the remaining 1/2 egg white.
9. Place the balls 2" apart on the baking tray and gently flatten with your fingers.
10. Brush all over with cold water then sprinkle with icing sugar. Lay an almond half on each biscuit.
11. Bake for 15-16 mins or until the tops of the biscuits are lightly browned.
12. Take care not to overcook as the biscuits should be crisp on the outside and moist and chewy on the inside.
13. Remove from the oven and use a spatula to take them off the silicone paper.
14. When cold store in an airtight tin.