Melted marshmallows become eerie cobwebs.
In large bowl, using electric mixer, beat sugar, 1/2 stick butter, and milk until smooth; spread between cake layers. In bowl, microwave chocolate and remaining 1 stick butter on high until melted; pour half of ganache over cake. Let stand for 10 minutes. Pour remaining ganache over cake. Let stand for 1 hour. In large bowl, microwave marshmallows on high until melted; stir until smooth. Wearing rubber gloves, pull marshmallows into thin cobweb strands; layer onto cake. Decorate with spider.