Preheat the oven to 325°F. In a medium saucepan, scald the heavy cream, then add the chocolate and stir until the chocolate has melted and the mixture is smooth. Remove the pan from the heat, stir in the light cream, and let cool.

Beat the egg yolks and sugar in a bowl until pale and thick enough to fall in a lazy ribbon from the beaters when they are lifted out, about 5 minutes. Fold in the cooled chocolate cream, followed by the espresso.

Divide the mixture among four 4- to 5-ounce teacups, ramekins, or other individual molds. Place the filled cups in a roasting pan. Set the pan on the bottom shelf of the oven and pull out the shelf halfway while keeping the pan level. Carefully pour enough boiling water into the pan to come halfway up the sides of the molds. Carefully push the shelf back into the oven.

Bake until the custards are just set at the edges but still slightly soft in the center, 20-25 minutes. Remove the molds from the water bath and let cool completely. Chill for a few hours or overnight.

Remove from the refrigerator 10-15 minutes before serving, garnished with a neat little dollop of creme fraiche and some chocolate shavings, if you like.