Melt the butter in a saucepan.
Add the courgettes and cook gently until tender.
Stir in the stock, salt and pepper to taste and herbs.
Reserve a few watercress sprigs for garnish and add the rest to the pan.
Bring to the boil, then cover and simmer gently for 20 minutes.
Puree the soup in an electric blender or rub through a sieve, then pour into a bowl.
Cool, then cover and chill thoroughly.
Just before serving, stir in the sour cream.
Serve garnished with crushed ice and the reserved watercress sprigs.