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 1/3 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, 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.
Cooks' note: Pudding can be made 1 day ahead and chilled, covered. Bring to room temperature before serving.