Place potatoes in a large pot filled with water over high heat. Bring to a boil and cook until potatoes are tender and can be easily pierced with a fork. (About 15 minutes) Drain the liquid out of the pot.
In a separate 6 quart pot or Dutch oven, over medium-high heat, add butter and cook until melted. Next add onions, chives, leeks and celery. Saute for a few minutes until the onions appear translucent and the celery is softened.
Add the cooked potatoes to the onion mixture, along with the chicken stock, milk, thyme, bay leaf, salt and pepper. Stir everything together and bring to a boil.
Turn the heat down to medium-low and let the soup simmer for 30 minutes.
Remove the soup pot from the heat and discard the bay leaf. Then puree the soup mixture with an emulsifier or blender. (If using blender, make sure to blend in small batches or you will have hot soup overflowing out of the blender. )
Let the soup cool down, then place in a containder and chill in the refrigerator overnight.
When ready to serve, add finely chopped chives with chive mousse and crouton (see recipes below). You can even garnish with an edible flower.
If you would like to serve hot: reheat over a low flame then add finely chopped chives with chive mousse and crouton.
Chilled Vichyssoise Soup Recipe https://whatscookingamerica.net/elleneaston-vichyssoise-soup-htm