Melt chocolate and butter together in a heatproof bowl over simmering water, stirring until smooth. Remove from heat and let cool for 5 minutes, then whisk in egg yolks one at a time until combined.
In a separate clean bowl, beat egg whites with salt until soft peaks form. Gradually add sugar and continue beating until stiff, glossy peaks form.
In another bowl, whip heavy cream with vanilla until soft peaks form.
Gently fold one-third of the egg whites into the chocolate mixture to lighten it. Then fold in remaining egg whites until just combined with no white streaks.
Fold in the whipped cream gently until mixture is uniform and airy. Divide among 6 serving glasses or ramekins.
Refrigerate for at least 3 hours or overnight until set. Before serving, top with whipped cream and chocolate shavings.