In a medium saucepan melt butter over a medium-high heat. Add the mushrooms and cook for 1-2 minutes or until the mushrooms start to get golden brown.
Add in flour and whisk together until well combined. Cook for 1-2 minutes to cook out the flour taste and making a roux.
Add chicken stock and milk, whisk until smooth, about 1-2 minutes.
Season with salt, pepper, garlic powder and onion powder.
Reduce heat to low and bring to a simmer. Cook until it begins to thicken, about 3-5 minutes.
Remove from heat and allow to cool completely.
Store in airtight container in refrigerator.
Use as you would condensed soup.