Preheat oven to 325 degrees. Bring the cream, espresso, vanilla, and chocolate to a simmer. Whisk until smooth.
In another medium bowl, whisk together egg yolks, sugar, and a pinch of coarse salt; add warm chocolate mixture in a slow stream, whisking constantly. Strain custard through a fine sieve into a 2-cup glass measuring cup.
Place six 4-ounce ramekins in the center of a baking dish lined with a towel. Divide custard among ramekins, and fill pan with enough boiling water to reach halfway up the sides of the ramekins. Cover with parchment paper. Bake until custard is set around edges but wobbly in center, 30 to 40 minutes.
Remove cups from water bath, and let custards cool on a wire rack for 1 hour. Cover and refrigerate for at least 4 hours and up to 2 days. Serve with whipped cream, if desired.