Melt the butter, add the onion, celery, carrot, and garlic. Cook until tender, about 8 minutes. Stir in the flour and cornstarch, cooking until bubbly, about 2 minutes. Add the stock, cheese, baking soda, and cream. Stir and blend until smooth and thickened. Add salt, cayenne, pepper, and bay leaf. Bring to a slow boil and let simmer 8 to 10 minutes. Remove the bay leaf. Test for consistency. May be thinned with a little milk if too thick. Garnish with the parsley and serve.