Clean and slice 3 large leeks, using only the white and light green parts. Rinse well to remove any dirt.

Peel and cube 4 large potatoes.

In a large pot, melt 2 tablespoons of butter over medium heat. Add the sliced leeks and cook until they start to soften, about 5 minutes, being careful not to let them brown.

Add the cubed potatoes to the pot, along with 1 liter of vegetable stock. Bring to a boil, then reduce heat and simmer for 20-25 minutes, or until the potatoes are very soft.

Use an immersion blender to puree the soup until smooth. Alternatively, you can use a blender, working in batches if necessary.

Stir in 100 ml of heavy cream, and season with salt and pepper to taste. Heat through.

Serve the soup hot, garnished with chopped chives or parsley if desired.