Line 2 baking sheets (each 14 by 17 in.) with cooking parchment, or butter sheets and dust with flour.
In a deep bowl, with a mixer at high speed, whip egg whites and cream of tartar to a thick foam. Continuing to beat, add sugar, 1 tablespoon every 30 seconds, then whip until meringue holds very stiff peaks.
Beat in vanilla. If using cooking 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). Pipe meringue onto baking sheets into ghostly shapes about 1/4 to 1/2 inch thick, 2 to 4 inches wide, and 4 to 6 inches long, spacing about 2 inches apart. To make eyes (noses and mouths, if desired) press chocolate chips lightly into meringue.
Bake in a 200 oven until meringues begin to turn pale gold and are firm to touch, 1 1/4 to 1 1/2 hours (1 to 1 1/4 hours in a convection oven); switch pan positions halfway through baking. Turn off heat and leave meringues in closed oven for 1 hour.
