In a medium bowl, whip the heavy cream until soft peaks form. Gradually add the sugar and vanilla extract, continuing to whip until stiff peaks form. Set aside.
In another bowl, combine the cooled coffee with the egg whites and cream of tartar. Using an electric mixer, beat until soft peaks form.
Gently fold the whipped cream into the coffee mixture until fully combined, being careful not to deflate the mixture.
Spoon the mousse into individual serving dishes or glasses.
Refrigerate for at least 2 hours to allow the mousse to set.
Before serving, top with chocolate shavings if desired.