Put the marshmallows into a heatproof bowl and microwave for 20 seconds - they will puff up slightly then collapse back down.
Mix well with a spoon until smooth and runny - if not runny enough to spread, return to the microwave for another 10 seconds then stir once more.
Place a spoonful on the flat side of half the mini cookies and half the standard cookies. Spread to the edges of the cookie, then arrange 8 pieces of liquorice string around the outside edge - bigger legs for the bigger spiders, smaller legs for the baby spiders.
Place a second cookie on top, flat side down, so that the marshmallow and legs are sandwiched between the two cookies, with the legs still sticking out.
Decorate the top of the cookie with white icing to create eyes and black icing for the mouth then leave to set for 20 minutes.