garnish: bittersweet chocolate curls (see cooks' note, below)
Put oven rack in middle position and preheat oven to 300°F.
Put chocolate in a heatproof bowl. Bring cream, milk, espresso powder (to taste), and a pinch of salt just to a boil in a small heavy saucepan, stirring until espresso powder is dissolved, then pour over chocolate, whisking until chocolate is melted and mixture is smooth.
Whisk together yolks, sugar, and a pinch of salt in another bowl, then add warm chocolate mixture in a slow stream, whisking constantly. Pour custard through a fine-mesh sieve into a 1-quart glass measure and cool completely, stirring occasionally, about 15 minutes.
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, covered, until cold, at least 3 hours.
*Available at some supermarkets, specialty foods shops, and The Baker's Catalogue (800-827-6836).
Cooks' notes: ·Pots de crème can chill up to 2 days. ·