1. Bring milk just to a simmer in a 1-quart saucepan over moderate heat. Remove pan from heat and whisk in espresso powder until dissolved. Keep warm, covered.
2. Whisk together granulated sugar, cornstarch, salt, and eggs in a 3-quart heavy saucepan until combined well. Add milk in a slow stream, whisking constantly. Bring to a boil over moderate heat, whisking constantly. Reduce heat and simmer, whisking, until thickened and registers 170 degree F on an instant-read thermometer, about 2 minutes. Immediately pour through a fine-mesh sieve set over a bowl (discard solids). Add chocolate, butter, and extract to bowl; let stand 5 minutes. Whisk until chocolate is melted and incorporated. Cover surface with waxed paper and chill until completely cool, about 1-1/2 hours. Spoon into crust, smoothing top, and chill, loosely covered, until filling is set, about 4 hours.
3. Spoon whipped cream into center of pie, sprinkle top with cinnamon and serve. Makes 10 servings.