Creamy Watercress Soup

0 Rinse the watercress well and roughly chop it 
1 In a large pot, melt the butter over medium heat and add the chopped watercress. Cook for 3 minutes: add the potato cubes, bay leaf, salt, and pepper, and add chicken stock to cover the watercress and potatoes.
2 Bring to a boil and lower to a simmer over medium-low heat for about 20 minutes. Partially cover the pan. 
3 Remove the bay leaf, and with a handheld mixer or in a blender and reduce a smooth, creamy puree.
4 Put the watercress puree back into the cooking pan, if you were using the blender, and add the heavy cream and stir well to integrate it. Lower the heat and warm up the soup, do not boil it.  Taste and season accordingly.
5 Serve with a dollop of heavy cream or sour cream.