Preheat oven to 200°F. Line 2 large baking sheets with parchment paper or silicone baking mats.
In the bowl of a stand mixer fitted with the whisk attachment, combine the egg whites, cream of tartar and salt. Whisk for about 1-2 minutes until it starts to become frothy, then gradually stream in ⅔ cup sugar. Continue to whisk until the mixture becomes shiny and thick, about 2-3 more minutes.
Add the vanilla extract; beat well.
Stop the mixer and remove the whisk attachment. Dip it into the meringue and pull it out straight up, then turn it over so the meringue forms a "peak". If it holds its shape {"stiff peaks"}, its ready. If not, continue to whisk and test every so often until it does.
Sprinkle in the remaining ⅓ cup of sugar, and carefully fold it in to the meringue as not to deflate all the fantastic whisking you've done.
Fit a piping bag with an open star tip. Fill the bag with meringue. Pipe about 2 inch swirls {like you are frosting a cupcake, except flat} as the "floor" of your cup, then pipe a "wall" on top of the base. This cup will help hold the custard filling later on.
Sprinkle the meringues with toasted coconut flakes, if desired. Bake the meringues for 2 hours. They should be dry and crisp, but check to make sure they are not browning. For truly crisp & set meringues, turn off the oven and allow them to sit for 3 to 4 more hours, undisturbed {or even overnight}.*
NOTE: If all this talk of piping bags and star tips intimidates you, skip it! You can scoop a dollop of meringue onto the parchment or silicone and make a "well" in the center using a spoon. This will make for a more rustic presentation, but it will still work just as well & be just as delicious! Never pass up a recipe just because it might not turn out looking perfect.