Place the ham in the bottom of a large pot over medium heat for a few minutes to get the flavor and juices out.
Pour in the buttermilk, evaporated milk, sour cream, and ham base.
Heat through, but do not bring to a boil.
Mix the cornstarch (I start with a tablespoon or two) with cold water and stir into the gravy. Keep stirring for a minute or two to keep it from settling at the bottom. Repeat this until it is the consistency you want.