In medium mixing bowl using an electric hand mixer whip together egg yolks and granulated sugar on high speed until pale and fluffy, about 2 minutes.
Warm 3/4 cup of the heavy cream and salt in a 2-quart saucepan on the stovetop over low heat (don't let it boil).
While whisking egg mixture slowly pour in warm cream mixture to temper egg yolks. Then pour combined egg yolk and cream mixture back into saucepan.
Cook over low heat, whisking constantly, until mixture thickens just slightly and reaches 160 degrees on an instant read thermometer (this usually takes about 3 - 5 minutes). If you notice any small lumps strain through a sieve and return to saucepan.
Off heat add in chocolate and vanilla. Stir well until chocolate is melted.
Pour mixture into a clean medium bowl, cover and chill, stirring about every 10 - 15 minutes until it reaches 70 degrees (or no longer warm), about 30 - 40 minutes total.***
Whip remaining heavy cream until very stiff peaks form.**** Fold whipped cream into chocolate mixture until combined.
Pipe or spoon into dessert cups. Chill 2 hours. Top with sweetened whipped cream if desired and garnish with shaved or grated chocolate.
*If you want whipped cream for topping I used about 1/2 cup cream, whipped with 1 Tbsp sugar.
**If you'd like it less intensely chocolatey you can use semi-sweet chocolate, if you'd like it more intensely chocolate and less sweet then you can reduce sugar to 2 Tbsp.
***Don't let chocolate mixture get too cold or it can start to harden and set and mixture will end up lumpy when folding in whipped cream.
****Using a chilled bowl is helpful when whipping cream, it whips faster and ends up fluffier.
Nutrition estimate is for mousse only, no added toppings.