Place a glass bowl over a saucepan of water and bring to the boil. Make sure the bowl isn't in contact with the water. Add chocolate pieces and remove from heat. Allow the chocolate to melt, without stirring.

Place egg yolks and sugar in an electric mixer and whisk for five minutes until the mixture is pale and creamy and doubled in volume. Combine with the chocolate.

Beat egg whites until stiff peaks form. Fold gently through the chocolate mixture in two batches.

Pour into individual glasses and cover with cling film. Refrigerate for one hour until set.