Pots de Crème

Combine the heavy cream and whole milk in a saucepan. Heat over medium heat, stirring occasionally, until the mixture just begins to simmer with small bubbles along the edges. Remove the pan from the heat immediately.
In a medium bowl, whisk the egg yolks until smooth. Slowly drizzle half of the hot cream mixture into the yolks while constantly whisking to prevent curdling. Pour the tempered yolk mixture back into the saucepan and heat over medium-low heat until the custard slightly thickens, about 2 minutes. Then remove from the heat and add the vanilla extract.
Stir in the chopped semisweet chocolate, adding the espresso powder and kosher salt along with it. Allow the chocolate to sit and melt for about 1 minute, then whisk until the mixture is smooth and fully combined.
Pour the custard through a fine-mesh strainer into a clean bowl, removing any clumps or bubbles. Divide the custard evenly among six 4-ounce ramekins.
Cover the ramekins and refrigerate for at least 2 hours, until set. Prior to serving, let them sit at room temperature for 15 minutes to soften slightly. Garnish with whipped cream if desired.