Using a cold whisk attachment, beat cold heavy cream in the cold bowl of an electric mixer, until soft peaks appear. Set it in the refrigerator until ready to use.
Preheat water in the double boiler over medium heat. Add chocolate, 1/4 cup of water, butter and vanilla extract.
Let it start melting and then stir with a silicone covered whisk until all melted and smooth. Take off heat and set aside.
In a small, heavy-bottom sauce pot, over medium-low (to low) heat, whisk egg yolks, 1/4 cup of water and 1/4 cup of sugar together. Cook stirring slowly but constantly, until the mixture reaches 160° (F). This could take up to 10 minutes but don't walk away or you will cook your eggs. Take off heat.
Cool and Combine:
Stir egg mixture into chocolate mixture until all combined and smooth.
Place some ice into a large mixing bowl. Set the sauce pot into the ice bath. Let chocolate cool, while slowly stirring, for 5-7 minutes.
Fold whipped cream into cooled chocolate mixture until all smooth. Take you time folding with a spatula, you don't want to whip or it will flatten.
At this time, you can divide chocolate mousse among dessert cups, cover air-tight, and refrigerate for at least 4 hours before serving.
Baking chocolate - make sure to use baking/cooking chocolate bar and choose either dark or semi-sweet. Get the chocolate from the baking isle, not the candy isle!
Heavy whipping cream – do not substitute this ingredient! It’s important to use heavy whipping cream to make whipped cream.
Adding liqueur - Feel free to stir in a tablespoon or two of amaretto, Kahlua, Bailey’s or crème de cocoa. Make sure to stir the liqueur into the egg yolk mixture, after you’ve brought it to temperature and before mixing it with the melted chocolate mixture.
Storing - Make sure to store chocolate mousse in an air-tight container in the refrigerator. Properly stored, it will last 5-7 days.