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