Line cookies in a single layer on a baking sheet with sides; set aside.
In a medium mixing bowl, sprinkle gelatin over ⅓ cup cold water to soften.
In a small saucepan over medium-high heat, dissolve granulated sugar in ¼ cup water, stirring. Remove spoon, brush inside edges of pan with water to remove sugar crystals and boil until the temperature reaches the soft-ball stage (238ºF) on a candy thermometer.
Add sugar mixture to gelatin and, using an electric mixer, whisk on medium speed for 3 min. Increase speed to high and whisk until soft peaks form, 8 to 10 min.
Transfer marshmallow to a piping bag fitted with a ½-inch-diameter plain tip, or coupler, and immediately pipe pointed mounds onto cookies (pipe just 5 ghosts at a time so marshmallow doesn’t set before you decorate).
Working quickly, add candy eyes and coat generously with crystal sugar.
Allow ghosts to set about 1 hour before serving.
Storage: store in a covered container for up to 2 days.