Cut the black licorice string into short lengths: 1/2 cm lengths for the noses and 1 cm lengths for the ears and tails.
Cut the 6 marshmallows in half lengthways. Using a skewer, poke two holes in the top end of each half for the ears, and poke two holes at the bottom of the front (uncut) side of each for the nostrils. Take the shorter 1/2 cm licorice pieces and stick them into the marshmallows where you've poked holes for the nostrils. Take the 1 cm pieces and stick them into the marshmallows where you've poked holes for the ears.
Spread the backs of two edible sugar eyeballs with a bit of buttercream icing. Attach to a marshmallow head for eyes, positioning above the nose. Repeat with remaining eyeballs.
Spread an even layer of buttercream icing over the cupcakes using a spatula or palette knife. Attach a marshmallow head to each cupcake.
Cover the cupcakes in mini marshmallows, sticking them to the icing by one of the flat sides. Leave space on the opposite side of the cupcake to the head to attach the tail. Poke a hole there using a skewer, then stick in the remaining 1cm licorice pieces for the tails.