Preheat the oven to 180c/350f. Line 2 cupcake trays with cupcake cases – you’ll need 16 cases. Place the bananas in a large bowl and mash them up with a fork. Add in the buttermilk, eggs, water and oil. Mix together with a hand whisk until combined. Whisk in the sugar, then add in the flour, cocoa powder, bicarbonate of soda and salt. Fold together until just combined (be sure to check there are no pockets of flour). The mixture will be wet and runny.
Divide the mixture between the cupcake cases and place in the oven to bake for 20 minutes (until a skewer inserted into one of the cakes comes out clean). Once cooked, take the cupcakes out of the oven. Leave to cool for 5 minutes in the trays and then place onto cooling racks to cool completely.
When you’re ready to decorate, melted the chocolate in separate bowls – either in a bain marie, or by melting in the microwave in 30-second bursts (stir in between each burst). Spoon three tbsp of the melted dark chocolate into a piping bag with a very fine tip. Spoon three tablespoons of the melted white chocolate into a piping bag with a very fine tip.
Take one of the cakes and spoon on a tablespoon of melted white chocolate – so it covers the top. Now pipe three concentric circles on top, using the dark chocolate. Finish with a dot of the dark chocolate in the middle. Use a toothpick to drag lines from the centre to the outside of the cake to look like a spider’s web. Six or seven lines will be fine.
Repeat with the remaining cupcakes – alternating between a white chocolate base and a dark chocolate base.
Serve immediately with the chocolate still warm, or leave to set before eating.