In a bowl, add flour, baking powder and salt. Sift and set aside.

In another bowl, add in sugar, butter and oil. Mix until they're very creamy. Add in vanilla extract and the eggs, 1 by 1, mixing to combine well after each addition. While still mixing, pour in the milk and continue mixing until fully combined.

Add in 1/2 of flour mixture and mix to combine. Do not over-mix to avoid hard, dense cake. Add the rest of the flour and mix to combine also. Again do not over-mix.

Take out 1/3 of batter and put it in a separate bowl. Pour in the melted chocolate and immediately fold everything together. Do not over-mix.

Pour the vanilla batter into a generously greased pan. Spoon in the chocolate batter on top. Using a knife, make some swirls but not too much as we don't want the batters to mix into one.

Bake at 160ºC preheated oven for 1 hour or until golden brown and fully cooked. Remove from the pan after 5 minutes and leave to cool completely on a rack. Slice and serve it.