Preheat oven to 350 degrees F.
Heat the cream, milk, sugar and espresso powder in a medium saucepan until it just comes to a boil, stirring to dissolve the sugar and espresso powder. Turn off the heat and add the chocolate. Stir until the chocolate is completely melted.
Whisk the egg yolks in a large bowl. Pour a small amount of the warm chocolate mixture into the yolks, whisking constantly. Pour the rest of the mixture into the yolks in a slow stream while whisking. Pour the mixture through a fine-mesh sieve into 4 ramekins.
Place the ramekins in a large roasting pan or baking dish. In a kettle or small pot, bring about 4 cups of water to a simmer. Place the roasting pan on a shelf in the middle of the oven and carefully pour the simmering water into the pan so that it comes halfway up the sides of the ramekins (this is referred to as a hot water bath). Cover the pan with foil and bake in the oven until the edges of the custards are set but the centers are still slightly wobbly, about 45-55 minutes. Remove from oven and place the ramekins on a wire rack to cool for an hour. Then refrigerate until cold, a few hours or overnight.
Serve pots de crème with a dollop of lightly whipped cream, if desired. Garnish with chocolate shavings and a dusting of espresso powder.