Throw the butter, heavy cream, corn syrup (or alternative), brown sugar and cocoa powder, in a saucepan over medium/low heat. Whisk to combine and continue whisking occasionally until it simmers. Turn down heat to low and simmer for a few minutes. Remove from the heat and whisk in the chocolate until completely melted and incorporated (the sauce will be super luxurious and shiny). Whisk in the vanilla and the salt. Proceed to sealing in mason jars per the link above or store in jar in fridge. Hot fudge sauce thickens when cooled, so to enjoy reheat in a pan or in the microwave.