Heat the oven to 200° and line a baking sheet with parchment paper. Use a black food writer to draw pupils on several button candies, then set them aside. With a mixer set on medium speed, beat the egg whites until soft peaks begin to form, about 4 minutes. Continue beating and add cream of tartar and vanilla extract. Gradually beat in sugar, then continue beating until stiff peaks form, about 6 minutes. Spoon the meringue into a ziplock bag and snip off a corner (we cut a 1/2-inch opening). Pipe 2-inch-tall ghosts onto the prepared baking sheet. Press a pair of candy eyes into each ghost. Bake the treats until they're hard but not browned, about 90 minutes. Turn off the oven and let the ghosts cool inside for 1 hour so that they'll be crisp. Remove them from the oven and let them cool completely on the pan before serving.