Preheat oven to 225 F.
Line two cookie sheets with parchment paper.
Beat egg whites and cream of tartar on low speed until frothy, then turn the mixer to medium speed and gradually add the sugar, one tablespoon at a time.
Turn the mixer to high and beat until the whites are very stiff and glossy, about 10 minutes. Slowly beat in vanilla.
Sprinkle the cornstarch evenly over the surface and beat 30 seconds more.
Hold the bag perpendicular to the cookie sheet and press the meringue out of the bag. Slowly lift the pastry bag straight up, still applying pressure, to make a ghost shape, then pull up quickly, without pressure, to create a peak.
Bake for approximately 1 1/2 hours. Note: Exact baking time depends on the indoor/outdoor humidity; the ghosts might need to bake a bit longer if the weather is especially humid.
Transfer to a wire rack to cool for 10 minutes or longer. They're ready when dry to the touch.