Preheat the oven to 180 °C/Fan 160 °C/350 °F/gas 4. Line a 12-hole muffin tin with paper cases.
Mix the soy milk and vinegar – it will thicken up quickly. Set it aside for later.
In a large bowl, combine all dry cake ingredients (flour, sugar, cocoa powder, baking powder and salt), then stir until everything is well mixed and there's no lumps. Pour in the soy milk and vinegar mixture, corn oil and vanilla extract. Whisk until a smooth batter forms, then evenly distribute it between the paper cases.
Bake for 15-20 minutes until the cakes are well risen. If you need to test, insert a cocktail stick into the middle of a cake – it should come out clean. However, if there's any cake batter on the stick return the cakes to bake for a few minutes more and test again.
Set the cakes aside on a rack until completely cooled.
Decorating the cakes
Sift the icing sugar in to a bowl, then add the water a little at a time and stir briskly until you have a thick icing that pours slowly.
Place the icing in a piping bag with a small tip and pipe a spiral on to a cake. Using a cocktail stick - or other small point - drag out straight lines from the centre of the spiral to the edge to form the cobweb pattern. Repeat until each cake is decorated.
Storage: These cakes are best served within 24 hours of baking, but they will keep well for up to three days in an airtight container in a cool place.