Preheat the oven to 500°. Trim away stems and leaves of the cauliflower, then remove the florets and trim into bite-sized clumps no more than 1 inch across at their widest point. You may need to halve larger florets through the stem. This should equal about 4 cups.
Combine florets with the melted butter in a baking dish; season generously with salt and pepper, and toss to coat. Drizzle in the broth. Roast until golden and tender, tossing every 5 minutes, about 20 minutes.
While the florets are roasting: In a large skillet, warm the olive oil over medium-low heat then add and sizzle the bacon and rosemary until golden, stirring frequently, 7 to 8 minutes. Add the garlic and stir for 15 seconds, then stir in the Vermouth, ¾ teaspoon salt, and plenty of pepper. Bring to a simmer, then remove from the heat.
Put a very large pot of water on to boil for the pasta; add 1 tablespoon salt. Cook the penne until al dente, then drain in a colander, reserving ½ cup of the pasta-cooking water.
Quickly add the cooked pasta to the pan with the bacon/Vermouth mixture and return the pan to medium-low heat. Turn the pasta with tongs to coat evenly, then add the roasted cauliflower, cold cubed butter, and enough of the reserved pasta water to make a nice juicy mixture (start with ¼ cup). Scatter with half of the Parmesan and parsley, and toss again until the butter cubes have just barely melted. Serve immediately on warm plates, dividing the remaining Parmesan and parsley over each serving.
Tip: Cut the butter into 4 cubes, place on a small plate, and hold in the refrigerator until just before finishing the pasta. The cold butter will make a rich emulsion with the Vermouth, starchy pasta water, and bacon mixture.