Place the butter into the saucepan and allow it to melt on medium heat.
Once the butter is melted, add in the sliced cremini mushrooms, season with salt and pepper, and sauté on medium to high heat until tender. About 3 to 5 minutes.
Meanwhile, separate about a ¼ cup of chicken broth, add the cornstarch to the ¼ cup and mix until there are no more clumps.
Once the mushrooms are slightly browned, and tender, add in the chicken broth, and bring to a gentil boil.
Add in the parsley, and slowly pour in the cornstarch mixture as you continuously mix. Continue to mix the gravy until the mixture thickens and begins to boil. Once the mixture reaches a full boil, reduce the heat to medium low, and continue to mix for 2 to 3 minutes. This extra cooking time will allow the raw cornstarch flavor to disappear. Remove from heat, allow to cool slightly, and serve.