Because it doesn’t have any stablizers (what helps give the storebought stuff its perfect texture), this hot fudge becomes quite firm in the refrigerator and a bit on the thin side when heated too vigorously. So I think leaving it out at room temperature for a few hours or lightly warming it is really the way to go for a smooth, spoonable hot fudge with a deep, dark chocolate flavor.
In a medium heavy duty saucepan, combine the cream, brown rice syrup, brown sugar, and cocoa powder. Bring to a boil, reduce the heat, and let simmer for 5 minutes, stirring occasionally to avoid clumping or burning.
Stir in the dark chocolate until fully melted. Increase the heat to medium-high and bring to a boil, stirring and scraping sides until bubbling vigorously, about 1 minute. Remove from heat and whisk in butter, vanilla and salt.
Strain sauce through a fine-mesh sieve into a heatproof bowl. Continue whisking until smooth. You want the consistency to be thick but pourable (but keep in mind it will thicken as it cools). Let cool to room temperature.
You can make this sauce up to 2 weeks in advance: simply store in the refrigerator, covered. Warm before serving. To reheat, microwave for 30-40 seconds until it’s pourable but still thick. Alternatively, warm in a heatproof bowl set over a saucepan of barely simmering water, stirring often and adding 1-2 teaspoons of water if needed to thin.