You'll need 3 baking sheets, covered with rice paper or baking parchment.
In a large bowl, mix together the ground almonds, 150g/5oz of the caster sugar and the candied orange peel.
Put the remaining caster sugar and 100ml/4fl oz water in a small saucepan and dissolve the sugar over a low heat without stirring.
Bring to the boil and, after a few minutes, test if the syrup has reached the correct 'soft ball' stage by dropping a little of it into cold water with a spoon – it should form a soft ball when rolled between forefinger and thumb.
Pour the syrup onto the ground almond mixture, adding 1 tablespoon icing sugar and 1 tablespoon flour.
Mix it well to form a soft dough.
Put in a bowl, cover with a clean, damp teatowel and chill for 30 minutes.
Preheat the oven to 180°C (350°F – Gas 4). In a small bowl, mix together 15g/.5oz of the vanilla-flavoured sugar, a pinch of bicarbonate of soda, 25g/1oz of the flour and 25g/1oz of the icing sugar.
Mix into the dough with half the egg white and knead until smooth.
Roll out the dough on a lightly floured work surface to a large 1cm/half inch thick square.
Using a sharp knife, cut horizontal lines spaced 5cm/2 inches apart, across the square.
Cut diagonal lines in the opposite direction across the square to form diamonds.
Knead together the trimmings, roll out and cut more diamonds.
Put the diamonds on the paper-covered baking sheets and dust the biscuits with the remaining vanilla-flavoured sugar.
Bake for 15 minutes until lightly coloured.
Leave the biscuits to cool completely before removing them from the paper.
Dust them with the remaining icing sugar before serving.