Preheat the oven to fan 170°C/340°F/ gas 5 and line a muffin tin with 10 muffin paper cases. Break the chocolate up and tip into a heatproof bowl, add the butter and liquid (milk, water or coffee). Place the bowl over a pan of gently simmering water and stir and leave the mixture to cool.
Mix the flour, bicarbonate of soda and both sugars in a bowl. Mix the egg in another bowl and stir in the soured cream. Next, pour this on the flour mixture and add the cooled chocolate. Stir until combined and don't overmix.
Spoon the mixture into the cases and place in the oven for 20 minutes until well risen. Lift out and palce on a wire rack to cool.
For the topping, break the dark and white chocolate into separate bowls and melt over a pan. Put two spoonfuls of dark chocolate in one piping bag and the same amount of white chocolate in a separate piping bag.
Spread a muffin with dark chocolate from the bowl, letting it run down a bit, then pipe four concentric cirles of white icing on top. Use a cocktail stick to drag through the circles at regular intervals from the centre to the edge to create a cobweb. Alternate the decoration by spreading the muffin with white chocolate and decorating with the dark chocolate. Decorate with jelly spiders.