Preheat oven to 325 degrees. Bring 1/2 cup plus 2 tablespoons cream, the espresso, and vanilla to a simmer; pour over chocolate in a medium bowl. Let sit for 3 minutes; whisk until smooth.
In another medium bowl, whisk together egg yolk, 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. Let cool completely, stirring occasionally, about 15 minutes.
Place 2 teacups in the center of a baking dish. Divide custard between cups, and fill pan with enough boiling water to reach halfway up the sides of the teacups. Cover tightly with foil; poke several holes in foil. Bake until custard is set around edges but wobbly in center, about 25 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. Before serving, whisk remaining 2 tablespoons cream to soft peaks, and dollop over pots de creme.