Put the chopped chocolate into a medium heat-proof bowl. Set over the pan of barely simmering water, ensuring the bottom of the bowl doesn’t come into contact with the water (or the chocolate will overheat) and allow it to slowly melt, stirring often until smooth (alternatively, pop into a microwave-safe bowl and melt in 20 second bursts, stirring between each - this is the quickest method but you have to watch closely and ensure it doesn’t overheat). Set aside to cool slightly.

Next, separate the eggs. Place the egg whites in a large bowl and whisk using an electric whisk until you have soft peaks. Add the sugar and whisk for another minute until glossy.

One at a time, add the egg yolks to the melted chocolate, mixing well after each. If the chocolate seizes, do not panic (it can happen and it is perfectly normal and fixable - so don’t stress), simply add a little boiling water, a tbsp at a time, and keep whisking until it comes back to being glossy.

Mix ⅓ of the whisked egg whites into the chocolate mixture and beat to combine. This lightens the chocolate and means it's easier to fold in the remaining egg whites without losing too much air. 

Carefully pour the loosened chocolate mixture into the remaining egg whites, gently folding to combine. Once no white remains, pour into a dish or individual glasses. Cover and chill in the fridge for at least 4 hours - or overnight.