Place a medium-sized bowl over a pan of simmering water and melt the chocolate.
Whilst the chocolate melts, wipe the lemon half around the inside of a big bowl to lightly coat it. Then, crack the eggs into the bowl, putting the egg yolks to one side.
Whisk the egg whites using an electric whisk, until they form peaks and are white and fluffy.
When the chocolate has melted, take off the pan of water and vigorously whisk in the egg yolks. Then, add a spoon of the whisked egg whites and whisk vigorously.
Finally, fold the rest of the whisked egg whites intothe bowl containing the chocolate mixture, making sure not to lose any of the air. Once combined, pour into glasses or ramekins and leave to chill in the fridge for around 2 hours or longer.