Preheat the oven to 180ºC/ 160ºC Fan/ Gas 4/350ºF. Line the bottom of 3 20cm (8 inch) springform baking tins with baking paper.
Melt the dark chocolate in short 30 second bursts in the microwave, stirring each time. Set aside to cool.
Cream the butter and sugar until lighter in colour and smooth.
Beat the egg yolks until they've doubled in volume.
Gradually add the egg yolks to the creamed butter and sugar, beating well.
Add the cooled chocolate and beat well to combine.
In a separate bowl, combine the flour, baking powder, bicarbonate of soda and salt.
Add a third of the flour to the creamed mixture and beat well, then add a third of the milk and beat. Repeat adding a third at a time with the remaining flour and milk.
In a separate bowl, beat the egg whites to soft peaks then fold the egg whites into the cake batter. Fold very gently so you don't knock the air out of the mixture!
Divide the mixture evenly between your tins. Bake for 28 mins or until a skewer inserted into the centre of each cake comes out clean. Leave to cool before filling.