Melt butter in a large pan over medium heat. Add mushrooms and cook for 5 minutes. Reduce heat to low.
In a bowl, whisk together milk and cornstarch. Slowly whisk mixture into pan with mushrooms.
Cook, whisking frequently, for 5-10 minutes or until mixture is bubbly. Whisk in salt, pepper, parsley and onion powder and cook for an additional 1-2 minutes. Remove from heat. Soup will thicken as it stands. Store in jar in fridge for up to 3 days.