Preheat the oven to 450 degrees . In a large bowl, toss the cauliflower with the rosemary and EVOO; season with salt. Spread the cauliflower on a rimmed baking sheet and roast, turning once, until tender and beginning to brown, 18 to 20 minutes.
Bring a large pot of water to a boil for the pasta. Salt the water, add the pasta, and cook, stirring occasionally, until al dente. Reserve 1 cup starchy cooking water; drain.
In a large skillet, melt 3 tbsp. butter over medium-low. Add the black pepper and swirl 1 minute. Add the reserved cup pasta water; simmer. Add the remaining 3 tbsp. butter and the pasta; remove from the heat. Toss, adding the cheese a few handfuls at a time, until a sauce forms and coats the pasta evenly. Add the roasted cauliflower; season. Serve in shallow bowls.