Beat the egg whites and citric acid powder on a slow speed until the air bubbles are evenly distributed, then gradually add the sugar to obtain foamy peaks. Gently add the confectioner's sugar and lemon zest using a spatula.
Use a spatula to spread out the meringue to a thickness of approx. 2mm on a silicone mat. Sprinkle with confectioner's sugar.
Bake at 250°F (110°C) for 60 minutes, then store in a dry place.
Note: This recipe cannot be frozen.