Use a pencil to trace around the rice cake on a piece of parchment paper.
Turn the paper over and pipe eight v-shaped legs around the perimeter of the rice cake.
Top with the rice cake.
Allow to set.
Pipe chocolate x's on the back of the candy eyes (or leave them as they are).
Remove the parchment paper from the rice cake, being careful not to pull off the spiders' legs (if this happens, use melted chocolate to 'glue' them back on).
Top with candy eyes.
Pipe a mouth with dark chocolate.
Insert two rod sprinkles for the fangs (or use flaked almonds).
Cover the top of the fangs with chocolate.
Allow to set.
Video
Notes
If you have any chocolate left over, pour it into an ice cube tray and freeze to use another day.