Make the corn starch slurry: In a small bowl, add cornstarch and 1 tablespoon of water. Stir together well and break up all the clumps. Set aside.
Heat a medium saucepan on medium-high heat. Melt butter in pan and then stir in mushrooms.Cook mushrooms until soft then slowly add vegetable stock, soy sauce and black pepper.
Bring gravy to a gentle simmer and cook for about 3 minutes or until mushrooms are tender. Add the cornstarch mixture and stir well so that no clumps form. Cook until the gravy is thick and remove from heat. Add additional salt to taste.