With the chocolate bar flat-side up, scrape a large kitchen knife over the surface to create chocolate curls (you only need enough to decorate the mousses before serving). Transfer to a plate and chill. Finely chop the remaining chocolate and put in a bowl set over a pan of gently simmering water, stirring every now and then, until melted. Alternatively, melt in the microwave in short bursts, to ensure it doesn’t burn. Set aside to cool for 10 mins.
Put the cream and peppermint extract in a bowl. Put the egg whites in another bowl with a small pinch of salt. Using very clean beaters, whisk the egg whites until they hold stiff peaks, add the sugar and briefly whisk again. Transfer the beaters to the cream and whisk until just holding its shape, then fold in 200g of the yogurt. Stir the egg yolks into the melted chocolate.
Add a large spoonful of egg white to the chocolate mix and stir to loosen the consistency. Add the remaining egg whites and the whipped cream. Gently fold everything together with a large metal spoon, trying to keep as much air in the mixture as possible. When there are no more visible streaks, transfer to eight serving glasses and chill for 2 hrs, or cover with cling film and chill for up to 24 hrs.
Before serving, top each mousse with a spoonful of yogurt, pomegranate seeds, chocolate shavings and mint leaves.