Vigorously whisk together yolk, 1 tablespoon sugar, flour, and a pinch of salt until well blended.
Heat milk and 3 tablespoons cream in a small heavy saucepan until hot but not boiling.
Add about one third of hot milk to yolk mixture in a slow stream, whisking constantly.
Add remaining milk, whisking, then transfer to saucepan. Bring to a simmer, whisking constantly, then continue to simmer, still whisking constantly, until thickened, about 1 minute.
Remove from heat and add chocolate, rum (to taste), and espresso powder if using.
Let stand until chocolate is melted, about 30 seconds, then whisk until smooth.
Transfer to a glass and chill, covered, at least 30 minutes to cool quickly to room temperature.
Just before serving, vigorously whisk remaining 3 tablespoons cream with remaining 1/2 teaspoon sugar in a small bowl until it holds soft peaks. Top pudding with whipped cream.
•Pudding can be made 1 day ahead and chilled, covered. Bring to room temperature before serving.
