Combine butter, cocoa, sugar, and cream in a saucepan over medium heat. Bring to a boil and boil for 7 minutes, stirring frequently.
Remove from heat; stir in vanilla and cinnamon. Cool slightly, then spoon over ice cream. Store any remaining sauce in the refrigerator.
Optional: for a smoother consistency, blend with an immersion blender after adding the vanilla and cinnamon.