Cream together the butter and sugar until light and fluffy, then mix in the egg and vanilla.
Sift in the flour and cocoa, mix until you have dough.
Split the dough in half and place one half between two sheets of baking parchment. Roll out until dough is around 4mm thick. Repeat with the other ball of dough, then chill the dough for 30 minutes.
Using a 6cm cutter, cut out 24 rounds. Place rounds on a baking sheet lined with baking parchment. Place back in the fridge for another 30 minutes.
Preheat the oven to 180°c then bake the biscuits straight from the fridge for 12-15 minutes until cooked through. Transfer to wire rack and leave to cool completely before assembling.
Melt the dark chocolate. Using a cocktail stick dip it in chocolate then draw in individual spider legs on baking parchment then allow to set.
Take one biscuit and dip in the remaining melting chocolate, then in chocolate strands to coat it. Decorate with two chocolate chips for eyes.
Place 1/2 tsp of Nutella in the centre of a plain biscuit and place 8 legs sticking out of the Nutella. Top with a decorated spider biscuit. Gently press together.