Preheat oven to 200°F with two racks close to the center.
Line two rimmed half-sheet pans with parchment.
Place the egg whites, cream of tartar, and salt into the bowl of a stand mixer fitted with the whisk, or into a large bowl if using electric beaters. Whip on medium-high for about three minutes, until stiff peaks form. (If you lift the beaters or a spoon out of the egg whites, the tips of the peaks that form should stand up without falling over.)
With the mixer running, add sugar a tablespoon at a time.
Turn machine up to high and beat for five minutes, until you have a big, glossy bowl of what looks like marshmallow fluff.
Sprinkle in cornstarch, vinegar, and vanilla, and fold in gently but thoroughly with a spatula.
Fit a large piping bag with a star tip. Using a skewer or a small paintbrush, paint several thin lines of gel food dye onto the inside of the piping bag, starting from the tip and going about halfway up the bag. You don’t really have to do a good job of this at all for the effect to work, so don’t stress about it.
Use the rubber spatula to transfer meringue from bowl into piping bag and twist the top to close.
Pipe bite-sized meringues (about an inch and a half in diameter) onto parchment-lined cookie sheets, leaving an inch or so of space between them.
Bake until dry and crisp all the way through, about two and a half hours.
Turn off the oven and let cookies cool completely in the oven to prevent cracking. You can prop the door ajar with a wooden spoon to speed this process if you like.