Beat in an electric mix egg whites with a tablespoon of sugar untill pale white.
Add the rest of the sugar, beat untill fluffy, then add powdered milk and beat untill incorporated.
Spread evenly the mix on a baking sheet (or make dots with sac- a- poche) and bake at 140°F untill hard.
Halfway engrave squares with a knife.
Store in an airtight container.