Preheat oven to 450 degrees and line a cookie sheet with parchment paper.
In a large bowl toss together the cauliflower, rosemary, olive oil and a pinch of salt. Spread on a baking sheet and bake for 18-20 minutes, turning once or twice until it begins to brown.
While the cauliflower is roasting, cook the bucatini. Boil in a large pot of salted water according to package directions until al dente. Reserve a cup of pasta water and drain the pasta.
In a large skillet, melt a tablespoon and a half of butter over medium-low heat. Add the black pepper and cook for a minute. Add about a half cup of the reserved pasta water and bring to a simmer. Add the remaining butter and melt.
Add pasta and remove from heat. Add the cheese a bit at a time until a sauce forms and coats the pasta.
Add the roasted cauliflower and toss with the pasta. Top with a little extra grated cheese.
Serve in bowls