Prepare Pastry Cream, but before whisking together the egg yolk mixture, place the milk and vanilla bean in a saucepan with 2 ripe but not soft bananas, coarsely chopped. Bring to a simmer, remove from the heat and let steep at least 1 hour (or transfer to a bowl and chill overnight). When ready to make the pastry cream, strain the milk through a fine-mesh sieve (discard the bananas or save them for another use; scrape the seeds from the vanilla bean, and stir them into the milk). Whisk the egg yolk mixture, heat the milk, and make the pastry cream as directed. If desired, stir into the hot custard (1 to 2 tablespoons dark rum, to taste). Just before using, fold 2 ripe but firm bananas, diced into the cold custard.