Step 1

Cook cream, brown sugar, brown rice syrup, butter, and ¼ cup water in a medium heavy saucepan over medium heat, stirring to dissolve sugar. Increase heat to medium-high and bring mixture to a boil. Fit saucepan with thermometer and cook, swirling pan occasionally (do not stir), until thermometer registers 220°, 6–8 minutes.

Step 2

Remove from heat and remove thermometer from saucepan. Add chocolate and stir until chocolate is melted and mixture is smooth. Bring to a boil over medium-high heat and cook, stirring and scraping down sides and bottom of saucepan with a heatproof rubber spatula to prevent scorching, until thickened and bubbling rapidly, about 1 minute. Remove from heat and whisk in cocoa powder, vanilla, and salt until smooth.

Step 3

Strain sauce through a fine-mesh sieve into a heatproof bowl—it will look oily and separated at this point, but don’t worry! Vigorously whisk 2 Tbsp. water into sauce to emulsify it; whisk until smooth and glossy. Whisk in more water a tablespoonful at a time until sauce is pourable but still thick.

Step 4

Do Ahead: Hot fudge can be made 2 weeks ahead. Cover and chill. Before serving, gently warm in microwave, stirring every 20 seconds, or in a heatproof bowl set over a saucepan of barely simmering water, adding water to thin if needed.