Preheat the oven to 225°F. Line two baking sheets with parchment., In the bowl of stand mixer, combine the egg whites, cream of tartar and salt. Beat until soft peaks form., With the mixer running at medium speed, gradually add the maple sugar. Beat until the mixture is stiff and glossy., Scoop by the quarter cup for large meringues, or pipe in walnut-sized mounds for small meringues. Sprinkle each meringue with a light dusting of additional maple sugar., Bake the meringues for 1 hour. Turn off the heat, and leave the meringues in the oven, with the door closed, until they're crunchy all the way through — at least 3 hours or preferably overnight., Once the meringues are dry, remove from the oven, and store airtight at room temperature.