Bring salted water to a boil and cook the pasta until al dente.
Steam the cauliflower until soft, about 10 minutes. Transfer to a blender along with a few tablespoons of the steaming liquid. Puree the cauliflower, adding more steaming liquid to thin the puree if necessary.
In a medium size casserole dish, combine the pasta and hot cauliflower puree. Add the milk, both cheeses, parsley, and season with salt and pepper to taste.
In a small bowl, mix the butter and breadcrumbs with your fingers until it resembles damp sand. Sprinkle across the top of the pasta mixture, then place the dish in the oven for about 20 minutes until the top is crispy and the sauce is bubbling.