Cook the cauliflower in a steam basket until super tender, about 8 to 10 minutes.
Meanwhile, heat the olive oil in a large stock pot set over medium heat. Add the onion, carrots, celery, salt and pepper and cook until fragrant, about 2 minutes.
Crank up the heat, add the chicken broth and water and bring to a boil; cook for 2-3 minutes and then add the dry rotini and continue cooking until the pasta is al dente, about 6-7 minutes; lower the heat and stir in the cooked chicken and frozen corn kernels.
When the cauliflower is nice and tender, take it off the heat and carefully transfer it to your blender or food processor. Add the milk and cornstarch and process on high speed until super smooth and silky; pour that right into the soup and stir until well incorporated.
Bring the soup back to a simmer and simmer for a full minute, then stir in chopped parsley and serve immediately.