Put the chocolate in a heat-proof bowl over a pan of simmering water, stirring every so often until melted. Make sure you don’t get any water near the chocolate. Remove from the heat and beat the butter into the chocolate with a whisk or spoon until smooth.
In a bowl that’s really clean – preferably washed in boiling water to remove any grease, and dried – whisk the egg whites until frothy but not stiff, using a mixer or an electric whisk on a medium high speed. Add half the sugar to the egg white and continue whisking on a low setting until the egg whites are stiff. Add the rest of the sugar and continue beating until the egg whites stiffen up even more, then mix in the egg yolks with a metal spoon.
Carefully stir half the egg mixture into the chocolate mixture, a whisk helps – then stir the rest in with a large spoon until well mixed in. Pour into a large serving dish and leave to set for a couple of hours, or overnight.