In a large sauce pan, melt butter over medium heat and swirl it around in pan to coat sides.
Add corn syrup and chocolate and stir until chocolate melts.
Add sugars, cream and salt and continue to cook over medium till mixture comes to a boil.
Using a candy thermometer, boil gently for 7 to 9 minutes, without stirring, until sauce is thickened and candy thermometer reaches 220-224º. Remove from heat.
Pour sauce into 4 cup Pyrex measuring cup.
Whisk in vanilla. Let cool a little while before serving.
Hot Fudge will last 2 weeks. Cool sauce completely before covering and chilling.