In a 2-quart saucepan over medium-high heat, bring the cream, syrup, brown sugar, cocoa powder, salt and half of the chocolate to a boil. Reduce the heat to medium-low or low (enough to maintain a low simmer), and cook for 5 minutes, stirring occasionally.
Remove from the heat and stir in the remaining chocolate, the butter, and the vanilla extract, stirring until smooth. Let cool for 20 to 30 minutes before using (it will thicken as it cools). Store in a jar or airtight container in the refrigerator for up to 2 weeks. To reheat, microwave for 30 seconds to 1 minute until it's pourable but still thick.
Notes
Lyle's Golden Syrup - I find this in the ethnic/international foods aisle in my supermarket; you can also order it online or substitute light corn syrup.
Dark Brown Sugar - You can substitute light brown sugar, but the dark gives the sauce a richer flavor.
Dutch-Processed Cocoa Powder - I get mine from Costco or Amazon, but some grocery stores carry it, as well. For the cleanest, richest flavor, do not substitute natural unsweetened cocoa powder.
Bittersweet Chocolate - I recommend using a 60% cacao for the best flavor, but you can go higher if you prefer a darker flavor or use semisweet chocolate if you want it a little bit sweeter. You can use bar chocolate and finely chop it, or you can use dark or semisweet chocolate chips.
Storage: Store leftovers of this hot fudge in an airtight container in the refrigerator for up to 1 month.
To Reheat: Microwave for 30 seconds increments, stirring after each, or until pourable but still thick.
To Freeze: Transfer the completely cooled hot fudge to an airtight, freezer-safe container and store in the freezer for up to 3 months. Transfer to the refrigerator to thaw overnight, then reheat as instructed above.
Flavored Extracts - Change up the flavor of the sauce by using different extracts; use ¼ teaspoon and reduce the vanilla extract to ½ teaspoon. Some ideas: peppermint extract, orange extract, and coconut extract.
Rum or Bourbon - Give your hot fudge a little kick by stirring in a tablespoon or so of your favorite liquor (give it a taste and add more if you'd like).
Flaky Sea Salt - Add a sprinkle of flaky sea salt at the end to balance out the flavors.
Peanut Butter Hot Fudge - Transform this recipe into a chocolate peanut butter version by stirring 2 tablespoons of creamy peanut butter into the mixture once the chocolate is melted.
More Flavor Ideas - You could also stir in some instant espresso (I would go with 1 teaspoon) for a mocha hot fudge or some ground cinnamon for a Mexican-inspired hot fudge sauce (use anywhere from ½ to 2 teaspoons depending on your preference).