Put on medium heat a large frying pan, add a couple of cooking water ladles, the butter, the camembert and 1/3 of the sliced mushrooms.
Then add some more cooking water and cook for a few minutes until the sauce is uniformly creamy. At that point toss in the pasta and another 1/3 of the sliced mushrooms, adding, if necessary, some more cooking water and stirring so that the sauce is creamy and well distributed. Adjust salt if necessary and grind some black pepper. Turn off the fire.