Combine the egg whites, cream of tartar & salt. Whisk for about 1-2 minutes until frothy; gradually stream in ⅔ cup sugar. Whisk until shiny & thick, about 2-3 more minutes.
Sprinkle meringues with toasted coconut flakes, if desired. Bake for 2 hours until dry & crisp, but not browned. If possible, turn off the oven & allow them to sit for 3 to 4 more hours before removing.