Preheat the oven to 180°C/350°F/gas mark 4. Whisk the egg with the icing sugar until very pale. Stir in the baking powder, ground almonds, ground rice, cardamom and grated lemon rind. Mix together thoroughly.
Grease a large baking sheet and put some icing sugar in a small bowl. Wet your hands with water, take up a little of the mixture and form into a ball the size of a large marble. Dip one side of the ball in the icing sugar and place it sugar-side up on the baking sheet. Continue like this, placing the balls well spaced out on the baking sheet as they spread quite a bit-you will probably have to cook them in 2 batches.
Bake in the centre of the oven for 10-12 minutes, until palest brown. Carefully ease the biscuits off the baking sheet with a palette knife and leave to cool on a wire rack.