For pesto: Puree basil, pine nuts and garlic with olive oil and salt in blender or processor. Transfer to bowl. Beat in cheeses, butter and pepper.
For soup: Slice leeks. Separate cauliflower into florets. Peel potato and thinly slice. Combine vegetables and broth in large saucepan and bring to boil. Reduce heat and simmer until vegetables are tender. Cool slightly. Puree in blender. Season with salt and pepper. (Can be prepared 2 days ahead. Cool completely and refrigerate.)
To serve, gently reheat soup. Ladle into bowls. Swirl 1 1/2 tablespoons pesto into each bowl.