Line a 9x13-inch baking dish with foil or parchment paper. Grease the foil/paper generously with butter, set aside.
In a dutch oven add the butter, brown sugar, granulated sugar, and corn syrup.
Place over medium heat and stir continuously until the sugars are melted then bring to a boil, about 10 minutes.
Once to a boil, very slowly add the heavy cream. Add about 1 teaspoon at a time and continuously stir; you want the mixture to be boiling the entire time. This can take up to 20-30 minutes to add all the cream, be patient with it.
Continue boiling the mixture white continuously stirring, scrape the bottom and sides of the dutch oven while doing so until the mixture comes to 240-245°F while measuring temperature with a candy thermometer.
Remove from heat and stir in the vanilla extract.
Immediately pour the caramel into the prepared baking dish. Let it sit out at room temperature until completely cooled.
Once cooled, place in the refrigerator until hardened, this will take 4-6 hours or overnight is better. Take the caramel out by lifting the foil/parchment paper out of the dish and then cut it into small pieces.
Sprinkle with sea salt if desired and serve immediately.