In a Dutch oven, cook leeks in butter with salt and pepper. Cover pan and cook on low heat for 10 minutes. Check often. Do not brown the leeks.
Add water, broth, and potatoes.
Bring to a low simmer and cook for 20 minutes.
Pour half of the soup mixture into a blender (carefully), puree, and return to the pan.
Add marjoram, parsley, and thyme. Add a few dashes of chili sauce, freshly ground pepper, and salt to taste. Serve hot.
Tip: When cooking leeks, only use the white parts. Take care not to overcook them. Overcooked leeks become slimy and unappetizing. Properly cooked leeks are tender but still offer resistance when pierced.
Variation: To make vichyssoise, a French version of the soup served cold, whisk in 1/2 cup heavy cream when returning soup to the pan after blending. Omit the hot sauce and add a pinch of nutmeg. Cool completely, then transfer to the refrigerator to chill. Serve cold.