6 oz good quality bittersweet chocolate (not unsweetened), finely chopped
1 1/3 cups heavy cream
2/3 cup whole milk
6 large egg yolks
2 tablespoons sugar
1 oz espresso, if desired
Put oven rack in middle position and preheat oven to 300 degrees.
Put chocolate in a heatproof bowl. Bring cream, milk, espresso (if using it) and a pinch of salt just to a boil in a small heavy saucepan, stirring, then pour over chocolate, whisking until chocolate is melted and mixture is smooth.
In another bowl, whisk together yolks, sugar, and a pinch of salt, then add warm chocolate mixture in a slow stream, whisking constantly.
Line bottom of a baking pan (large enough to hold ramekins) with a folded kitchen towel and arrange ramekins on towel. Poke several holes in a large sheet of foil with a skewer. Divide custard among ramekins, then bake custards in a hot water bath , pan covered tightly with foil, until custards are set around edges but still slightly wobbly in centers, 30 to 35 minutes.
Transfer ramekins to a rack to cool completely, uncovered, about 1 hour. (Custards will set as they cool.) Chill until cold, at least 3 hours.
To serve, garnish with a dollop of creme fraiche and espresso beans or chocolate covered espresso beans, if desired. Fresh berries, fresh mint leaves, bacon, or salted caramel sauce all make for a great garnish with the creme fraiche as well.