Whisk brown sugar, cocoa powder, cornflour, espresso powder (if using), cinnamon, salt and 1/2 cup of the milk in medium saucepan.
Turn on heat to medium-low; pour remaining milk into sugar mixture while whisking. Bring to a simmer. Let simmer, whisking constantly, until slightly thickened (about 2 to 3 minutes).
Beat egg lightly in small bowl. Pour in 1 cup of the hot-milk mixture while whisking. Pour egg mixture back into saucepan. Cook over medium-low heat, stirring, for 2 minutes.
Remove pan from heat. Add chocolate. Whisk until smooth. Stir in vanilla. Pour into 4 serving dishes. Chill, covered, for at least 2 hours.