1. Place the chocolate in a microwave-safe bowl and melt in a microwave, stirring after each 10-second increment, until smooth, about 1 1/2 minutes. Let cool slightly.
2. Place the egg yolks & 1/2 cup sugar in a medium bowl & whisk until pale yellow. Whisk in the olive oil, coffee, zest, if using, & salt until combined. Add the melted chocolate & whisk until smooth.
3. Beat the egg whites in the bowl of a stand mixer or with a hand mixer until soft peaks form. Gradually sprinkle in the remaining ¼ cup sugar & beat just until stiff peaks form. Add a generous spoonful of the egg white mixture to the chocolate mixture. Stir firmly until completely incorporated. Pour the chocolate mixture into the bowl of egg whites. Gently fold with a large spoon or rubber spatula until completely combined.
4. Divide the mousse into six 6-ounce ramekins & cover with plastic wrap. Refrigerate overnight. Serve with a dollop of whipped cream & shaved dark chocolate.
Recipe courtesy of Epicurious