Preheat the oven to 210°F (100°C).
Line a baking tray with a silicone baking mat.
In a large bowl, gently whisk the egg whites until they begin to foam, then whisk vigorously until the whites start to firm up. You can use an electric mixer but do not whisk for too long. Do not let the whites become firm.
Add powdered sweetener little by little, and keep whipping for about 1-2 minutes.
Pour the mixture into a piping bag with a large nozzle or use a spoon for a rustical look (and fewer dishes, wink wink). Pipe (or spoon) the meringues onto the baking mat.
Put the tray in the oven. Every 20 minutes open the oven door for a few seconds to remove moisture. We are making the "dry" meringue cookies here.
After 50 minutes, turn off the oven and allow the white cookies to cool for several hours in the oven. Ideally overnight.