Fold a kitchen towel into quarters and place in the bottom of a straight-sided, 11-inch sauté pan and fill said pan with an inch or so of water. Place over medium-high heat and bring to a bare simmerYou should see steam and tiny bubbles, especially around the edge of the pan., about 190°F.
Place the chocolate in a heat-safe bowl — either metal or heavy glass will do — and park right on top of the towel. Reduce the heat to medium-low and stir the chocolate with a rubber or silicone spatula off and on until it’s almost melted — 3 to 5 minutes. Then move the bowl to a dry towel on the counter and stir the chocolate until completely melted.
Whisk the yolks into the chocolate one at a time, followed by the whole egg and the salt. The mixture will look gnarly after the first two yolks, but it will smooth out once everything is in. Allow to cool to room temperature, then...
Beat the egg whites using the whisk attachment(s) on your stand or electric hand mixer on medium-low speed until foamy —1 minute. Increase the speed to medium-high and keep beating until the whites form stiff peaks — about 1 minute more. (You’ll know you’re there when you pull a whisk straight out of the foam, hold it facing up and the points or “peaks” of the egg whites don’t fall over.) If the whites start to look dry or chunky, you’ve gone too far and will need some new egg whites.
Using the whisk, stir a third of the egg whites into the chocolate mixture. Then, switch to a large rubber spatula and gently fold the remaining whites into the chocolate in two doses, rotating the bowl as you go. Stop while your mousse still has tiny streaks of chocolate and white throughout.
Chill the mousse until it begins to stiffen, about 45 minutes, then divide between four Champagne coupes or small bowls. Allow to warm on the counter 5 minutes prior to serving.