Put the almonds, icing sugar and flour in a pestle and mortar or food processor and grind to a fine powder.
Whisk the egg whites.
Add caster sugar and beat until you have a stiff shiny mixture. Fold in the almonds.
Put the dough into a pastry bag with a small round nozzle. Line baking sheets with aluminum foil. Force the dough through the nozzle into 1 1/2-inch rounds. Sprinkle each macaroon with about 1/4 teaspoon confectioners' sugar.
