Fill a large saucepan with an inch or two of water and set it over medium heat until water comes to a low simmer.
Place a medium bowl over the simmering water and place chocolate inside. Stir gently until chocolate is fully melted.
6 ounces milk chocolate
Once fully melted, whisk the butter into the melted chocolate, 1 tablespoon at a time. Remove chocolate mixture from heat.
4 tablespoons unsalted butter
Stir in the salt, vanilla, and coffee until completely incorporated.
¼ teaspoon salt, 1 teaspoon vanilla extract, 2 tablespoons strong brewed hot coffee
Whisk in yolks, one at a time, making sure that each is fully incorporated before adding the next. Set chocolate mixture aside.
4 large eggs
Stir egg whites in a clean mixing bowl set over a saucepan of hot water until slightly warm, 1 to 2 minutes; remove bowl from saucepan.
4 large eggs
Beat with an electric mixer set at medium speed until soft peaks form. Raise mixer speed to high and slowly add sugar, beating to soft peaks.
2 tablespoons granulated sugar
Whisk a quarter of the beaten whites into chocolate mixture to lighten it, then gently fold in remaining whites.
In a separate bowl, whip the heavy cream to soft peaks. Gently fold into mousse.
1 cup heavy cream
Spoon portion of mousse into six to eight individual serving dishes or ramekins.
Cover with plastic wrap and refrigerate to blend flavors, at least 2 hours, before serving.
Optional: When ready to serve, whip heavy cream on high speed in a clean bowl until stiff peaks form. Serve chocolate mousse with a dollop of whipped cream and a sprinkle of chocolate shavings.
chocolate shavings