In a medium saucepan, combine heavy cream, butter, brown sugar, sugar and salt over medium-low heat. Bring to a simmer for about 1 minute.
Add chopped chocolate and stir until melted. Remove saucepan from the heat and whisk in the cocoa powder.
Return saucepan to low heat and bring to a simmer whisking constantly until glossy, about 30 seconds. Whisk in vanilla and remove from heat. Transfer to a bowl, sauce boat or jar with a lid for storage. Serve warm.
To reheat, warm in a saucepan over low heat and frequently stir until warm. Alternatively, microwave the sauce on 50% power level stopping to stir every 15 seconds until warm.
Notes
Store hot fudge sauce in an airtight container in the refrigerator for up to 4 months.