Set sous vide to 167°F
Microwave chocolate, cream, sugar, espresso powder, and salt until chocolate is melted and mixture is thoroughly combined
Whisk in egg yolks and vanilla until combined
Strain chocolate mixture through fine-mesh strainer into liquid measuring cup, then divide evenly among widemouthed Mason jars
Seal jars and lower into water bath until fully submerged, cover and cook for 60 minutes
Transfer jars to wire rack and let cool to room temperature, refrigerate until chilled for at least 4 hours
Let pots de crème sit at room temperature for 20 minutes before serving
