In a medium saucepan, heat heavy cream, sugar, and butter over medium heat, stirring until the butter melts and the sugar dissolves (~3-4 minutes).
Reduce heat to low and whisk in chopped chocolate and cocoa powder until completely smooth (~2-3 minutes).
Stir in vanilla extract and salt. Continue cooking for 1-2 minutes until the sauce thickens slightly.
Remove from heat and let cool for 5 minutes before serving. The sauce will thicken further as it cools.
Transfer to a glass jar and store in the fridge for up to 2 weeks. Reheat gently before use.
