In a medium saucepan over medium heat, mix cider vinegar, molasses, dry mustard, butter, cayenne pepper, Worcestershire and brown sugar. Bring to a boil. Mix 4 teaspoons cornstarch with 4 teaspoons cold water and slowly add to the sauce; simmer 1 minute. The sauce will thicken a little but will remain mostly thin as this is the type of sauce it supposed to be (it will also thicken a bit more when it cools). If you think the cornstarch became clumped in any way within the sauce, strain before pouring into a squeezable bottle.