Beat the egg whites in a bowl; when they are frothy, add half the caster sugar and continue to beat. When they start to become stiff, add the remaining sugar and continue beating to make a thick meringue mixture. Using a spatula, fold in the sieved ingredients. Use large, gentle movements for at least 2 minutes, working the mixture from the edges of the bowl towards the centre. The mixture should form a ribbon when it falls from a spoon.