Add the butter to the skillet. Once melted, add the mushrooms. Sprinkle with salt and cook, stirring occasionally, until tender and golden, 7 to 10 minutes. Add the garlic and cook for 20 to 30 seconds. Add the cream cheese and stir until melted and smooth. Whisk in 1 cup of the reserved pasta water, a little at time, until smooth, followed by the Parmesan. Pour the sauce over the noodles and broccoli, and toss to coat. If it’s not quite creamy enough, add a little of the remaining pasta water. Taste and adjust seasoning.