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.
Mix the flour and baking powder, stir into the mixture alternately with the milk. Split the mixture into 2 portions. Mix the vanilla paste into one half of the mixture, transfer to the prepared cake tin.
Place the chocolate in a thin-sided bowl and suspend over a gently simmering bain-marie, ensuring that the bowl does not touch the water. Melt the chocolate and stir until smooth. Stir the chocolate into the other half of the sponge mixture along with the rum or milk, spread on top of the pale sponge mixture. Create a marble effect by running a fork through the mixture in a spiral motion.
Approx. 60 mins. in the lower half of an oven preheated to 180°C. Remove from the oven, allow to cool slightly, tip out onto a cooling rack and leave to cool completely.