Preheat the oven to fan 170C/ conventional 190C/gas 5 and line a muffin tin with 10 paper muffin cases. Break the chocolate into a heatproof bowl, add the butter and liquid. Melt in the microwave on Medium for 30-45 seconds (or set the bowl over a pan of gently simmering water). Stir and leave the mixture to cool.
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 pour this on the flour mixture and add the cooled chocolate. Stir just to combine - don’t overmix or it will get tough.
Spoon the mixture into the cases to about three quarters full. Bake for 20 minutes until well risen. Loosen the edges with a round-bladed 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 out of greaseproof paper (or cut 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 (or over a pan as in step 1). Put 2 spoonfuls of dark chocolate in one bag and the same of white chocolate in the other.
Working with one muffin at a time, spread with dark chocolate from the bowl, letting it run down a bit, then pipe four concentric circles of white chocolate on top. Using a small skewer, drag through the circles at regular intervals, from the centre to the edge, to create a cobweb effect. Repeat with four more muffins. On the remaining five, spread over the white chocolate and decorate with the dark. Best eaten the day they’re made - even better while the chocolate’s soft.