Cut the cartons off 6 single-serve chocolate ice cream cups and place each ice cream round on a graham cracker square. Freeze until firm, about 20 minutes. Meanwhile, beat 4 egg whites and a pinch of cream of tartar with a mixer until foamy, 30 seconds to 1 minute. Gradually beat in 2/3 cup sugar, then beat on high speed until stiff shiny peaks form, 4 to 5 minutes. Spread the meringue over the ice cream, then brown with a kitchen torch and freeze until serving.