Do not crowd the mushroom pan. A mushroom is nearly all water, and 400 g piled up will steam in its own liquid and stay pale instead of browning. Two batches is quicker than one bad one.
The pasta keeps drinking. The penne cooks in the soup, so the pot goes on thickening after it comes off the heat and again overnight. Loosen it with water rather than stock, or it ends up too salty.
Temper the cream. A ladle of the liquid only, no solids, stirred into the cream and then poured back. Double cream is forgiving; single cream splits, so if that is what you have, warm it slowly and keep the pot off the boil afterwards.
No vinegar. A dried-mushroom soup is soured at the end, a fresh one is not. The optional yoghurt is the gentler version of that.