Heat a pot over medium or medium high heat, with about 2 – 3 inches of water. Bring it to a simmer. This will be used as a double boiler / bain marie, for the yolks.
Place the chocolate, butter, and salt in a microwave safe bowl, and microwave it in 20 – 30 second intervals, stirring in between. When the chocolate chips are about 75 – 80% melted, stir the chocolate butter mixture until all of the chocolate is melted. Set aside until needed.
In a large metal bowl, place the egg yolks, water, vanilla, and 1/2 of the sugar. Whisk to combine. Place the bowl over the pot of simmering water. Heat the egg yolks to 170°F while constantly whisking. Once heated, remove from the heat.
Add the butter and chocolate mix into the egg yolk mix. Mix until well combined and then set aside.
Whisk the egg whites until they are glossy and you have mid peaks. Mid peaks is beyond soft peaks, but not quite at stiff peaks.
Add about ¼ of the egg whites to the chocolate yolk mix, and mix it in to "loosen" the chocolate base.
Add the rest of the egg whites and carefully fold it in, until it's about 70 – 80% mixed through (some egg white streaks are still visible). Set aside.
Add the heavy cream into the same bowl that you used to whisk the egg whites. Whisk until you have soft peaks.
Add this cream to the chocolate mousse base and fold it in until mixed through.
refrigerate for at least 1 hour if you will be piping
Place the chocolate mousse in a shallow dish, or divide between 6 individual serving dishes.
Cover with plastic wrap and let it chill for at least 4 hours