Add the chocolate, butter, and salt to a microwave-safe bowl. Microwave in 20-second bursts, stirring between each burst, until melted and smooth. Do not overheat. Set aside to cool slightly.
In a large bowl, whisk the egg yolks with half of the sugar (25g or 2 tbsps) using a hand or stand mixer fitted with the whisk attachment on medium speed for about 3 minutes, until pale, thick, and creamy. Set aside.
In a clean medium bowl, whip the egg whites with the remaining sugar (25g or 2 tbsps) using a hand or stand mixer fitted with the whisk attachment (see note 1) on medium speed for 3–5 minutes, until stiff peaks form. Set aside.
In another medium bowl, whip the cold cream with the vanilla using a hand or stand mixer fitted with the whisk attachment on medium speed until soft peaks form. The cream should be lightly thickened but still loose (see video for demonstration). Set aside.
Gently fold half of the warm, runny chocolate (see note 2) into the egg yolk mixture using a rubber spatula, then fold in the remaining chocolate until just combined.
Next fold in half of the whipped cream to loosen the mixture, then gently fold in the remaining cream until almost combined.
Lastly, gently fold in the whipped egg whites in two additions, mixing until just combined and being careful not to deflate the mousse.
Spoon the mousse into serving glasses or ramekins (see note 3) and smooth the tops. Refrigerate for at least 4 hours, or ideally overnight, until fully set. Cover if needed to prevent absorbing fridge odours.
Serve chilled, topped with whipped cream and grated chocolate if desired.