In kettel or large saucepan, combine cauliflower, potatoes, 4 cups milk, and salt. Bring to boil. Cover and simmer for 30 minutes. Strain, reserving liquid. Mash vegetables. Stir in cooking liquid. Return to pan. Add remaining milk. Bring to boil.
Off fire, add butter, stirring until butter is melted and well blended. Place fried diced bread in bottom of soup tureen. Pour soup over it. Serve hot.