Preheat the oven to 180°C / 350°F / Gas mark 4.
Line a cupcake/muffin tin with 10 paper cupcake cases.
Break the chocolate into a bowl, add the butter and milk.
Melt in the microwave on medium for 30-45 seconds
Stir and set aside.
Mix the flour, bicarbonate of soda and both sugars in a bowl.
Beat the egg in another bowl and stir in the soured cream, then add this in with the flour mixture and the cooled chocolate.
Stir just to combine, don’t over mix.
Spoon the mixture into the cases to about three quarters full.
Bake for 20 minutes until well risen.
Loosen the edges with a knife, let them sit in the tins for a few minutes, then lift out and cool on a wire rack.
For the topping, make two piping bags by cutting the ends off two clean plastic bags.
Break the dark and white chocolate into separate bowls and melt in the microwave on medium for 2 minutes.
Put 2 spoonful’s of dark chocolate in one bag and the same of white chocolate in the other.
Working with one cupcake at a time, spread with dark chocolate from the bowl, letting it run down a bit, then decorate the tops creating the cobwebs.
Repeat with four more cupcakes.
On the remaining five, spread over the white chocolate and decorate with the dark.