Combine flour, salt and baking powder and mix well.
In a separate bowl, cream butter with sugar. Add vanilla sugar. Then add eggs one a time and mix well. Add soured cream and mix.
Sift flour mixture into butter mixture, mixing as you go. You should end up with smooth cake mixture.
Divide the cake mixture in half. Add cocoa to one half.
Grease 1.5 litre loaf tin (or similar size). Add 2 tablespoons light-coloured mixture to the centre of the tin, top with 2 tablespoons chocolate mixture. Continue alternating colours until both mixtures are used up.
Bake in the oven for about 60 minutes, or until a skewer inserted in the centre comes out clean.