Break the dark chocolate into small pieces and place in a heatproof bowl set over a pot of barely simmering water. Stir occasionally until melted, smooth, and glossy. Remove from heat and let it cool for 5 minutes—it should be warm, not hot.

In a clean mixing bowl, add the egg whites and a pinch of salt. Beat on medium speed until foamy, then gradually add the sugar. Increase to high speed and whip until stiff peaks form. The whites should hold their shape and not slide if the bowl is tilted.

Lightly beat the egg yolks in a separate bowl. Slowly stir in the cooled chocolate until the mixture is smooth and evenly combined.

Add one-third of the whipped egg whites into the chocolate mixture and fold gently with a rubber spatula to lighten it. Then fold in the rest in two batches, using slow, sweeping strokes. Stop once the mixture is fluffy and no streaks remain.

Spoon the mousse into serving glasses or ramekins. Tap lightly to level, then cover and chill for at least 2 hours. Just before serving, top with whipped cream and a few fresh raspberries if you like.