In a medium saucepan bring the heavy cream, sugar, brown sugar, corn syrup, chocolate, and butter to a boil over medium heat, whisking until the mixture comes to a boil. Cook, whisking constantly, until the mixture thickens and feels like fudge sauce, 5 minutes. Remove from the heat and whisk in the vanilla. Pour the fudge into a shallow glass dish or bowl to cool (The fudge pours best when it’s not too hot and not too cold, at the consistency of honey; fudge can be made ahead and reheated in a water bath of lukewarm water to the desired consistency).