In pot, melt butter over medium heat; cook leeks 2 minutes. Add potatoes; cook 5 minutes. Add broth; bring to boil. Simmer until fork-tender, 15 minutes. Remove one-third potatoes; reserve. Puree remaining soup with 1 cup water. Stir in chicken, reserved potatoes, cheese and 3 tbsp. sour cream. Top with remaining sour cream and cayenne.