Melt the butter in a large pot or Dutch oven over medium heat. Add leeks and cook, stirring often, for about 6-8 minutes or until beginning to soften.
Add potatoes, chicken stock and 1 teaspoon salt and bring to a boil. Reduce heat to low and simmer, covered, for 15-20 minutes, or until potatoes are cooked through.
Remove from heat and puree with an immersion blender. Alternatively, allow to cool a few minutes and transfer to a blender and process until smooth, then return to the pot.
Stir in cream and add salt and pepper, to taste. Serve, garnished with fresh herbs if you like.