Bring the heavy cream, light corn syrup, dark brown sugar, cocoa powder, and salt to a boil in a heavy saucepan over medium heat, stirring constantly until the sugar dissolves and the mixture is smooth.

Reduce heat and add the chopped chocolate. Cook for 5 minutes, stirring frequently, until the chocolate is fully melted and the sauce is thickened.

Remove from heat and stir in the butter and vanilla extract until smooth and well combined. Cool slightly before serving.