Step 1Place the cream, milk, espresso powder, chocolate, brown sugar and salt in the top of a double boiler. Cook over simmering water, stirring often, until the mixture is scalding and the chocolate has melted, about 5 minutes. Stir until smooth.
Step 2Whisk the egg yolks lightly in a small bowl, then gradually whisk in half of the chocolate mixture. Pour into the double boiler with the remaining chocolate mixture. Cook, stirring, until the mixture has thickened and mounds when spooned, about 15 minutes. Remove from the heat. Stir in the vanilla, then pour into 4 pots de creme cups, demitasse cups or small custard cups, dividing evenly. Cool, uncovered, then chill until cold, covered, about 3 hours or overnight.
Step 3To serve, top with small dollops of whipped cream. Serve immediately.