Line round pizza pan with parchment paper; set aside. Melt butter in large saucepan set over low heat; stir in 5 cups marshmallows, stirring constantly, for 3 to 4 minutes or until completely melted. Stir in vanilla. Remove from heat. Immediately stir in 8 cups popcorn until coated. Spread onto prepared pizza pan into irregular round shape, about 1-inch thick, to resemble spider web. Refrigerate for about 15 minutes or until firm.
Meanwhile, in microwave, melt remaining marshmallows for about 20 seconds or until melted. Using spatula, string strands of melted marshmallow over popcorn spider web to create cobweb effect.
In heatproof bowl set over saucepan of hot, not boiling, water, melt 3/4 cup of the chocolate chips; let cool slightly. Pour chocolate over remaining popcorn in large bowl, folding gently to coat. Spread mixture on waxed paper–lined baking sheet, separating into 8 small clusters. Affix 2 candy eyes on each popcorn cluster. Refrigerate for 10 to 15 minutes or until set.
Melt remaining chocolate chips. Cut licorice into 1-inch lengths. Use small spoon, dab circles of chocolate onto spider web. Affix chocolate-coated popcorn onto chocolate circles. Affix licorice lengths to chocolate popcorn clusters to resemble spider legs.
Refrigerate for 15 minutes or until set. To serve, cut into smaller pieces.