First, make whipped cream and set it in the fridge until ready to use: Using a cold whisk attachment, beat cold heavy cream in the cold bowl of an electric mixer, until soft peaks appear.
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 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 degrees (F). This could take up to 10 minutes but don't walk away or you will cook your eggs. Take off heat.
Stir egg mixture into chocolate mixture until all combined and smooth.
Place some ice into a large mixing bowl. Set the sauce pot in 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 and refrigerate for at least 4 hours before serving.