Make cookies one day in advance. Prepare dough and roll out following recipe directions. Use cutters from set to cut one skull, four small bones, four large bones and four candy corn shapes. Bake and cool cookies.
Decorate cookies. Prepare Color Flow icing following recipe directions. Tint portion black. Reserve some white. Use tip 3 and full-strength white icing to outline skull and bone cookies. Use thinned white icing in cut disposable decorating bag to fill in cookies. Use tip 3 and full-strength black icing to outline candy corn cookies. Use thinned black icing in cut disposable decorating bag to fill in cookies. Let set overnight.
Use tip 3 and full-strength black icing to pipe eyes, nose and mouth on skull. Use tip 6 and full-strength white icing to pipe finger and toe bones on candy corn cookies.
Make brownies. Prepare batter following recipe directions. Bake and cool brownies. Use knife to cut brownie into 2 in. squares.
Assemble treat. Pile brownies on foil-wrapped foam core, cut to 16 in. x 32 in. x 1/2 in. Use brownie to prop skull cookie. Insert bone cookies in pile between brownies, and add hand and feet cookies. Place gummy worms between brownies. Crumble a couple of brownies into pieces and sprinkle crumbs around brownie pile.