Preheat the oven to 350 F. Line 2 muffin pans with 16 paper liners.
Put the chocolate, butter and sugar in a heatproof bowl and melt, stirring frequently. You can melt the chocolate in the microwave (about 3 minutes, stirring after each minute) or over a pan of warm water (make sure the base of the bowl doesn't touch the water). Leave to cool slightly.
Whisk together the eggs, vanilla and sour cream. Stir this into the cooled chocolate.
Put the flour, cocoa powder, and salt in a strainer or sifter and sift over the chocolate mixture, then fold in, along with the milk chocolate (if using). Spoon into the paper liners, filling to around 2/3 full (an ice-cream scoop is good for this).
Bake in the preheated oven for 18-20 minutes, until the muffins are risen and just firm to the touch. Let cool for 5 minutes, then transfer to a cooling rack.
Now for the decoration: Melt the chocolate in a heatproof bowl over a pan of simmering water.
Using a palette knife or flexible spatula, cover each cake with some of the melted chocolate.
Arrange eight licorice strips for the spider's legs and stick a mallowmar in the center of each. finish with licorice allsorts and chocolate candy eyes. You can use a blob of writing icing to stick the candy on the licorice allsort.