Make sure to wash the chopped leeks really well; then put them into a pot with the chicken stock, set over medium heat and bring to a boil.

Cover with a lid, turn the heat down to a simmer, and cook for 20 minutes.

Add the potatoes, cover again, and cook for another 20 minutes. Turn the heat off, and leave to cool to room temperature.

Put the leek and potato mix in a blender along with the buttermilk and some salt and pepper. I like to use white pepper to avoid having black flecks in the soup, but this is purely an aesthetic choice.

Blend until very smooth, taste and add more salt and pepper if you need.

Chill the soup until very cold.

Pour into bowls, and swirl a tablespoon of crème fraîche inside. Top with a few slices of avocado, some shredded cold chicken, and scatter over the chives.