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Melt the chocolate in a heatproof bowl over a pan of simmering water, making sure the bowl doesn’t directly touch the water. Once melted, allow it to cool for 5-10 mins, then stir in the yogurt.
Whisk the egg whites until stiff, then whisk in the sugar and beat until stiff again. Fold the whites into the chocolate mix – loosen the mixture first with a spoonful of egg white, then carefully fold in the rest, keeping as much air as possible.
Put berries into small glasses or ramekins, then divide mousse on top. Chill in the fridge until set.