Melt butter in a pot on the stovetop.
Add the sugar to the butter and allow it to heat up and cook. It will eventually start to boil and bubble up. Temperature should be around 300 to 350 degrees, so be very careful when working with caramel. (If making candy, the temperature should generally be higher, so use a candy thermometer to monitor).
Pour in the cream and continue to stir. Follow by adding in the cornstarch only if using milk. NOTE: The cream is sufficient on its own to thicken the caramel sauce. Allow mixture to cook and thicken. This should take only minutes—continuously observe and stir as needed. Then remove pot from heat.
Stir in the vanilla. Allow the caramel to sit and cool completely before using.
Whenever possible use organic ingredients.