Potato leek spinach soup

4 large potatoes (I used large bakers, all I had in the house)
 2 large leeks, white and tender green parts only
 3/4 pound mushrooms (I used plain white button mushrooms)
 3/4 pound fresh spinach
 salt
 butter buds (optional)
 skim milk (optional)
 Peel and coarsely chop potatoes.  Trim leeks, slice in half longwise and 
 separate under running water to remove all sand and dirt, then coarsely 
 chop.  Clean and quarter mushrooms.  Place all in large pot and just 
 cover with water.  Add salt, about 2 teaspoons, and bring to boil.  
 Simmer until potatoes are tender, about 20 minutes.  Ladle 1/2 of cooked 
 potato mixture into food processor and add half of the spinach.  Process 
 until smooth and thick.  Repeat with rest of potatoes and spinach.  Add 
 about 1 Tablespoon butter buds for flavor and thin with skim milk if needed.
 Serve hot.  Makes about 6-8 servings (although 2 of us ate at least half 
 of it at one sitting, with just fresh crusty french bread along side!)
 
 Notes on variations from Mollie's original version:  I thought I used too
 much water at first and removed some before I processed the soup.  It was
 then thicker than I wanted, so I thinned it with the milk.  The milk and
 butter buds were not in Mollie's original version.  She used watercress,
 but suggested that spinach, or any other green, would also be good.