Preheat oven to 200° with racks in upper and lower third. Line 2 baking sheets with parchment paper, or butter the baking sheets and dust with flour.
In a deep bowl, with a mixer on high speed, whip egg whites and cream of tartar until thick and foamy. While beating, slowly add sugar, a few tablespoons at a time every 30 seconds or so, then whip until meringue holds very stiff peaks.
Beat in vanilla. If using parchment, smear a little meringue on the underside of each corner to make it stick to baking sheets.
Spoon meringue into a pastry bag fitted with a 1/2-inch plain tip (or spoon into a gallon-size heavy plastic food bag, then cut off 1 corner to make a 1/2-inch-wide opening). Hold bag perpendicular to baking sheet and pipe, with even pressure, to form 2-in.-tall mounds of meringue. (If you make them taller, they'll fall over.) If you want to give the ghosts arms, put some meringue in a smaller bag, snip a smaller opening (or use a smaller tip), and pipe arms onto ghosts.
Bake meringues until they're firm to the touch and no longer sticky but not yet beginning to brown, 1 to 1 1/2 hours; switch pan positions halfway through baking. Turn off heat and leave meringues in closed oven for several hours or overnight.
Put chocolate in a 2-cup glass measure and heat in a microwave until melted, 80 seconds, stopping and stirring every 20 seconds.
Spoon chocolate into a small piping bag with a tiny tip (or a resealable plastic bag with a tiny corner cut off). With steady but slow pressure, squeeze chocolate onto ghosts to create a variety of expressions.