In a small skillet cook 1/4 cup of the sugar over moderate heat, stirring with a fork, until it is melted and a golden caramel, add carefully 1/4 cup water and simmer the mixture, stirring, until the caramel is dissolved. Remove the skillet from the heat and stir in the chocolate until it is melted. In a bowl whisk the yolks and the whole egg with the remaining 1/4 cup sugar and add the chocolate mixture, whisking.
Whisk the scalded milk in a steady stream and add the espresso powder, vanilla, and a pinch of salt. Strain the custard through a fine sieve into another bowl, skim the froth, and divide the custard among six 2/3-cup pot de crème pots or ramekins.
Put the pots in a baking pan, add enough hot water to the pan to reach one third up the sides of the pots, and cover the pots with their lids or cover the pan tightly with foil.
Bake the custards in the middle of a preheated 300°F oven for 30 to 35 minutes, or until they are just set, let them cool completely, uncovered, and chill them, covered, for 3 hours, or until they are cold. The custards may be made 1 day in advance and kept covered and chilled.
Garnish each custard with a rosette of whipped cream and a chocolate coffee bean.