Melt the chocolate in the microwave in a microwaveable bowl, stirring every 30 seconds until just melted. Whisk the egg whites to soft peaks, scatter in the sugar and whisk again until the mixture forms stiff peaks when you lift the whisk out (around 30 seconds).
Add a big tablespoon of the egg white to the melted chocolate and mix vigorously, working fast so the chocolate doesn’t cool and harden. Then fold in the remaining egg whites carefully using a spatula or large metal spoon. Spoon into 4 glasses or ramekins and set in the fridge for 2-3 hours. Serve each with a dollop of crème fraiche and shavings of dark chocolate.