Place softened butter and sugar into large mixing bowl. Mix, using hand mixer or stand mixer on medium speed for 1-2 minutes.
Beat egg whites until stiff peaks form.
Add egg yolks to butter.
Add flour alternately with the stiff egg white, then add milk and salt and mix until the mixture is smooth.
Divide the mixture between 2 bowls.
Break chocolate and place into a heatproof bowl. Sit over a pan of barely simmering water (a bain marie) and allow the chocolate to melt, stirring occasionally.
Stir melted chocolate into the mixture in one of the bowls.
Take 2 spoons and use them to dollop the chocolate and white cake mixes into the tin alternately.
Bake the cake for 50-55 mins at 160 degrees until a skewer inserted into the centre comes out clean.