Combine sugar and egg in a medium bowl; stir with a whisk. Add milk and next 4 ingredients (through salt); stir to combine. Transfer milk mixture to a medium saucepan; cook over medium heat 14 minutes or until thick and bubbly, stirring constantly. Remove from heat; stir in chocolate, butter, and vanilla. Cool 10 minutes before serving.
I do not think I cooked it long enough. I did not want it to burn. It was thick and bubbly when I removed it form the stove, but it did not hold up well. Strong coffee flavor. I will make it again and use instant coffee instead of espresso and will definitely leave it on the stove longer. We enjoyed it cool the next day.
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