In a large saucepan over low heat, melt together the chocolate, butter, and sugar, stirring constantly until smooth.
Stir in the milk and cook for 2-3 minutes more, until the mixture is heated through and slightly thickened.
Remove from the heat and stir in the vanilla, cinnamon, and salt until well combined.
Pour into mason jars and store in the fridge for up to 2 weeks.
To reheat: microwave sauce in a glass container for 2 minutes, stirring halfway through, until warm and pourable, then spoon over ice cream, cake, or cookies.
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Notes
Nutrition information is an estimate, calculated automatically. Values vary with brands, substitutions and portion sizes.