Preheat the oven to 190ºC/ 170ºC Fan/ Gas 5
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.
In a jug, combine the milk and vanilla extract.
Add half the flour to the creamed mixture and beat well, then add half the milk and beat. Repeat 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. Please note: I don't grease and line my cake tins using the traditional method, I spray them with Wilton's Cake Easy and have never had a problem with cakes sticking.
If using shallow layer tins, your cakes will probably be baked after 20 minutes. If using 2 tins then they'll take around 30 minutes. The cakes are ready when a skewer inserted into the middle comes out clean.
Leave the cakes to completely cool while you make the buttercream.