Melt butter in a medium-to-large pot without browning it. Trim the leek, quarter the white stalk lengthwise and dice it. Add leek and salt to the pot and sweat gently rather than frying.

Turn heat to low and cook leeks about 20 minutes until soft. Peel and dice potatoes.

Add chicken broth, potatoes and bay leaves. Cover and cook 45 minutes, then remove bay leaves.

Dice chicken sausage. In a pan, sauté sausage with some of the fresh herbs until browned; remove from heat.

Add cream and buttermilk to the leek-potato mixture and blend until creamy. Add sausage and remaining herbs and blend briefly. Add chives and pulse to combine.