How to make
Mix the sugar and butter, add the eggs that you've beaten well in advance in a separate container. Stir well and mix the baking powder and salt with the flour. Start adding to the mixture a little flour, then a little milk, while alternating between them and stirring constantly.
Set aside 1/3 of the mixture and to it add the chocolate that you've melted ahead of time. To the other mixture, add the vanilla. In a cake form that you've buttered and floured ahead of time, pour a large spoon of the white mixture, a spoon of the chocolate one and repeat until you run out.
Put the cake to bake in a preheated oven at 360°F (180 °C) for approximately 50-60 minutes. Leave it aside to cool for about an hour and invert it onto a platter.