Using a stand mixer, whip cream until medium peaks form, about 30 seconds. Then put into the refrigerator if you are working in a warm kitchen. Otherwise, keep it out on the counter.
Chop both types of chocolate into small chunks, then put into a large microwave-safe bowl. Heat chocolate in microwave for 2-3 minutes at 50% power, stirring every 30 seconds, until melted. Stir in salt.
Heat corn syrup in microwave for 10-15 seconds, until almost boiling.
Whip yolks using a hand mixer, drizzling in corn syrup. Continue to whip until the eggs and syrup thicken to the point that if you lift the beaters, the mixture pours down in thick ribbons (ribbon stage). This will take around 3-4 minutes.
Pour the egg yolk mixture over the melted chocolate and do not stir.
Wash the egg beaters, and get out a clean bowl.
Add egg whites to the bowl and whip to firm peak, about 30-40 seconds.
Stir together the chocolate and egg yolk mixture, adding in a scoop of the reserved whipped cream.
Fold the rest of the whipped cream into the chocolate mixture.
Stir in the port (if using), then fold in the whipped egg whites.
Put mousse into single serving cups (or whatever serving dish you want to use). Chill for 3 hours.
Remove from the refrigerator 15-30 minutes before serving.
Top with whipped cream and chocolate shavings to serve, if desired.
Notes
Expert Tips: Work as quickly as you can with this recipe—the mousse will come together best if the chocolate does not cool too much. You want to be able to combine the melted chocolate with the rest of the ingredients before the melted chocolate gets too cold. If it seems like the chocolate is starting to harden as you mix in the whipped cream or egg whites, stir together as quickly as possible. For this recipe, I suggest the use of a hand mixer for whipping the egg yolks and syrup. You can use a stand mixer, but I find that it’s sometimes hard for the beater to reach only 2 egg yolks in the bottom of the mixer bowl.This recipe contains raw eggs. If you are concerned at all about not cooking your eggs, look for pasteurized eggs to use in this recipe. Make sure you separate your eggs carefully—getting a little bit of egg yolk into the whites will make the whites much harder to whip into peaks. (Getting a little bit of white into the yolks is fine!)Storage Tips: Store chocolate mousse in its serving dish/es, tightly covered with plastic wrap. Wait to top with whipped cream and chocolate shavings until ready to serve. Mousse will stay fresh for 3-4 days.