Set sous vide temperature to 167 degrees.
In a small saucepan on low heat, melt chocolate, cream, sugar, espresso powder, and salt. Stir until chocolate is melted and mixture is well combined. Take off the heat.
Whisk egg yolks in a small bowl. Add a small ladle full of chocolate mixture and stir well. Repeat once more. This will temper the egg yolks. Stir in vanilla. Stir the egg yolk mixture into the chocolate mixture.
Strain the chocolate mixture through a fine sieve into a 4 cup liquid measure. Divide evenly among 6 4 oz. wide mouth Mason jars. Gently tap the jars on the counter to remove air bubbles, then seal; do not overtighten.
Gently lower jars into water bath until fully submerged. Cover and cook for at least 60 minutes or up to 75 minutes.
Transfer to wire rack and let cool to room temperature, about 1 hour. Refrigerate until chilled, at least 4 hours. Let Pots de Crème sit at room temperature for 20 minutes and serve.