Peel the shallot and chop finely along with the chives.
Melt the butter in a large saucepan over a medium heat.
Add the chopped shallot and cook for about 3 minutes, stirring regularly, until the shallot is translucent.
If you are using frozen corn, add it to the pan and cook for a minute stirring.
Next, add the flour and stir for a minute to combine.
Gradually add the milk, stirring, (add drained tinned sweetcorn if you are using at this point), as the mixture comes to a simmer and thickens.
Remove from the heat.
Put the mixture into a blender or use an immersion blender to blend the soup.
Stir in the stock and bring back to a simmer.
Add the cayenne, chives and crab and stir gently to combine.
Heat for a further minute and serve.
Serve as it is or with a drizzle of cream and a sprinkling of extra chives and cayenne pepper.