Place the butter in a bowl, whisk in the sugar and salt. Add the eggs one at a time and continue to whisk until the mixture becomes lighter in colour. Stir in the milk.
Mix the flour and baking powder, stir into the mixture. Divide the cake batter into two portions, mix the vanilla into one half of the batter.
Place the chocolate in a bowl, pour over the water and leave to stand for approx. 2 mins. Carefully drain the water, stir the chocolate until smooth, stir into the other half of the batter.
Transfer the light batter into the prepared tin, spread the dark batter on top. Using a fork, stir in a spiral motion to create a marble effect through the light batter.
Approx. 60 mins. in the lower half of an oven preheated to 180°C. Remove from the oven, allow to cool slightly, remove from the tin and leave to cool completely on a rack.