Melt the butter over medium heat in a large sauce pan. Add mushrooms and saute until golden, about 5 minutes. Reduce heat to low.

In a separate bowl, whisk together the cornstarch and milk until smooth. Slowly add the mixture to the mushrooms in the pan, whisking constantly. Cook, whisking frequently, over low heat until bubbly.

Whisk in salt, pepper and parsley. Remove from heat. The soup will thicken slightly as it cools.