Place butter in 2 1/2- to 3-quart microwave-safe bowl. Microwave on HIGH 1 minute. Add leeks and bell peppers; microwave on HIGH 3 minutes.
Drain juice from clams into microwaved vegetables, reserving clams. Stir in potatoes, broth and thyme. Cover with plastic wrap, venting one corner. Microwave on HIGH 20 minutes.
With slotted spoon remove 4 cups cooked potatoes; set aside. Pour contents of bowl into container of electric blender; add milk, and holding lid down tightly, blend until smooth. Return mixture to bowl. Stir in reserved clams and potatoes, corn and cayenne; season with salt and pepper. Microwave on HIGH 3 minutes until heated through. If desired, pass bowls of shredded Cheddar cheese, chopped parsley or crumbled cooked bacon to stir into soup.