Add the butter to the skillet. Once melted, add the mushrooms. Sprinkle with salt & cook, stirring occasionally, until tender & golden, 7 to 10 minutes. Add the garlic & cook for 20 to 30 seconds. Add the cream cheese & stir until melted & 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, broccoli & chicken, toss to coat. If it’s not quite creamy enough, add a little of the remaining pasta water. Taste & adjust seasoning.