For the pots de crème: Place chocolate in medium heatproof bowl; set fine-mesh strainer over bowl and set aside.

Whisk yolks, sugar, and salt in medium bowl until combined; whisk in heavy cream and half-and-half. Transfer mixture to medium saucepan. Cook mixture over medium-low heat, stirring constantly and scraping bottom of pot with wooden spoon, until thickened and silky and custard registers 175 to 180 degrees on instant-read thermometer, 8 to 12 minutes (see photos, below). Do not let custard overcook or simmer.

Immediately pour custard through strainer over chocolate. Let mixture stand to melt chocolate, about 5 minutes. Whisk gently until smooth, then whisk in vanilla and espresso. Divide mixture evenly among eight 5-ounce ramekins. Gently tap ramekins against counter to remove air bubbles.

Cool pots de crème to room temperature, then cover with plastic wrap and refrigerate until chilled, at least 4 hours or up to 72 hours. Before serving, let pots de crème stand at room temperature 20 to 30 minutes.

For the whipped cream: Using hand mixer or standing mixer fitted with whisk attachment, beat cream, sugar, and vanilla on low speed until bubbles form, about 30 seconds. Increase speed to medium; continue beating until beaters leave trail, about 30 seconds longer. Increase speed to high; continue beating until nearly doubled in volume and whipped cream forms soft peaks, 30 to 45 seconds longer.

Dollop each pot de crème with about 2 tablespoons whipped cream; garnish with cocoa or chocolate shavings, if using. Serve.