Prepare cake as directed on package.  Pour into 2-8 inch square aluminum foil pans coated with cooking spray; bake at 350 degrees F for 30-35 minutes.  Cool in pans on wire rack.

 

Combine 4 cups powdered sugar and 5-7 Tbsp milk in a large bowl; spread a thin layer evenly over each cake.  Combine 1 cup powdered sugar, cocoa and 2 Tbsp milk; spoon into a small zip-lock bag.  Close bag securely and snip a tiny hole in bottom corner of bag.  Carefully pipe concentric circles onto each cake.  Pipe straight lines at intervals from the inner circle through the outside circle, forming a web design.  Garnish with plastic spiders.