Line a flat baking tray with baking paper.
Mix the ground almonds with the caster sugar, 100 g (3½ oz) of the icing sugar, the baking powder and the orange zest in a bowl.
Whisk the egg whites to soft peaks and combine with the almond mix, using a wooden spoon. Mash the mixture together to make a sticky, wet mass.
Mould oval shapes to 6 cm (2½ in) long x 4 cm (1½ in) wide x 2 cm (¾ in) thick onto a spoon. Slide the ovals off the spoon carefully with your thumb, and reshape any indentations. Put them onto the baking tray, allowing room for them to spread. Sprinkle the tops with 25 g (1 oz), or half of the remaining icing sugar. Leave the biscuits on the tray for 2-3 hours at room temperature to dry them a little before baking.
Preheat the oven to 140°C (275°F/Gas 1). Bake for about 30 minutes or until they are lightly golden and a little firm on the outside. The insides will still be soft. Remove from the oven. Cool slightly before sprinkling with the remaining 25 g (1 oz) of icing sugar. Cut the rice paper around the bottom of the cakes. They will keep for about a week stored in an airtight container.