1. Preheat your oven to 350°F (175°C) and grease your chosen cake pan.
2. In a bowl, whisk together cake flour, baking powder, and baking soda.
3. In a separate large bowl, beat butter, oil, and sugar until light and fluffy.
4. Add eggs one at a time, mixing well after each addition.
5. Stir in buttermilk and vanilla extract to the wet ingredients.
6. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
7. Separate about 30% of the batter into another bowl.
8. Mix melted chocolate or cocoa powder into the smaller portion of batter to create the chocolate mixture.
9. Alternately spoon the vanilla and chocolate batters into the prepared pan.
10. Use a knife or skewer to gently swirl the batters together to create a marble effect, being careful not to overmix.
11. Bake in the oven for 30-40 minutes, or until a toothpick inserted in the center comes out clean.
12. Let the cake cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.
13. If desired, top with chocolate frosting or ganache once the cake has cooled.
14. Serve and enjoy your homemade marble cake!
15. Store leftovers at room temperature for 2-3 days, in the refrigerator for up to 5 days, or freeze for up to 3 months.