Add the cream and milk to a saucepan over medium heat. Whisk occasionally until it starts to simmer with bubbles along the edges, then remove from the heat.
Whisk the egg yolks in a medium bowl. While continuously whisking, very slowly pour half of the hot cream mixture into the bowl to temper the eggs. Don't pour too fast or the eggs will curdle.
Return the tempered egg mixture to the saucepan and stir together over medium-low heat for 2 minutes, or until the custard is slightly thickened. Turn off the heat.
Add the chopped chocolate, espresso powder (optional) and salt to the custard. Let the chocolate melt for a minute, then whisk until completely smooth.
Strain the mixture through a fine mesh strainer into a bowl with a pour spout, to remove any clumps.
Pour into small bowls or ramekins and refrigerate until set, about 2 hours.
When ready to serve, let the pots de crème stand at room temperature for 15 minutes, to soften slightly. Garnish and serve.
