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 ½ 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.
Assemble & Bake
Take out ⅓ 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 10 inch 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 (320ºF) 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!