Set up a double boiler with simmering water. Combine chocolate and butter in the bowl and melt, stirring occasionally, until smooth and glossy. Remove from heat and let cool slightly, about 5 minutes. The mixture should be warm but not hot to touch.

In a separate bowl, whisk egg yolks with 1 tablespoon sugar until pale and thick, about 2 minutes. Gradually whisk the warm chocolate into the yolks until fully combined. The mixture will be thick and shiny.

In a clean bowl, beat 3/4 cup cold heavy cream with an electric mixer on medium-high speed until it holds soft peaks, about 2-3 minutes. Do not overbeat. Gently fold the whipped cream into the chocolate mixture in two additions until no streaks remain.

In a clean, dry bowl, beat egg whites with salt and vanilla on medium speed until foamy, about 1 minute. Increase speed to medium-high, gradually add remaining 2 tablespoons sugar, and beat until whites hold firm, glossy peaks, about 3-4 minutes. They should stand straight when you lift the beaters.

Stir one-third of the beaten whites into the chocolate mixture to lighten it. Gently fold in the remaining whites in two additions, cutting down through the center and lifting up the sides. Stop when just combined - a few streaks are fine. Overfolding deflates the mousse.

Divide mousse among 6 ramekins or glasses. Tap gently on the counter to release large air bubbles. Cover and refrigerate at least 4 hours, preferably overnight, until fully set and spoonable.

Just before serving, beat remaining 1/2 cup cream with powdered sugar to soft peaks. Dollop onto each mousse and add chocolate shavings if desired. Serve cold.