Preparation

• Put the chocolate and coffee in a heavy saucepan. Heat gently over low heat until the chocolate starts to melt; set aside. In a medium saucepan, heat the milk until bubbles begin to form around the edge; don’t let it boil. In a small mixing bowl, whisk the egg yolks with the sugar and a pinch of salt just until well blended.
• Whisk half of the hot milk into the egg yolks. Return the yolk mixture to the milk in the saucepan. Cook slowly over low heat, stirring continuously with a wooden spoon, until the custard thickens slightly and coats the back of the spoon (enough so that a finger run through the sauce will leave a clean trail). The time it takes for this thickening to occur will vary. It may take as little as a few minutes or as many as 20, but once the thickening begins, it happens quickly; watch carefully or the eggs will coagulate. Check with a thermometer; don’t let the temperature exceed 160°F. Immediately pour the custard through a strainer over the partially melted chocolate. Whisk until smooth. Transfer to a large mixing bowl and let cool, stirring occasionally, until the mixture is about 96°F; if you dab a bit on your lip, it should feel just slightly cooler than your lip. Don’t let the mixture set or the mousse won’t be smooth. (If it cools too far, set it over a pan of hot water briefly and whisk until smooth.)
• Beat 2 cups of the heavy cream until it holds soft peaks. With a rubber spatula, gently fold half of the whipped cream into the chocolate mixture until no white streaks remain. Gently fold in the remaining whipped cream. Spoon the mousse into six goblets or one large bowl. Cover and chill for at least 3 hours or up to two days.
• To serve, beat the remaining 1/2 cup heavy cream until it holds soft peaks. Dollop a little on each serving and scatter with the chocolate shavings, if using.