Preheat the oven to 350°F (180°C). Spray two 9-inch round cake pans with nonstick cooking spray, line the bottom of the pans with parchment paper, and set aside.
In a large mixing bowl, whisk the cake flour, baking powder, baking soda, and salt together until well combined. Set aside.
In a large bowl or measuring cup, whisk the buttermilk, oil, and vanilla extract together until well combined. Set aside.
In a separate mixing bowl, whisk the cocoa powder and water together until well combined and no lumps remain. Set aside to cool.
In the bowl of a stand mixer fitted with the paddle attachment or in a large mixing bowl using a handheld mixer, beat the butter on low speed until smooth, then slowly mix in the granulated sugar.
Once all of the sugar is added, increase to medium speed and continue mixing for another 4 to 5 minutes or until the mixture is light and fluffy.
Mix in the eggs, one at a time, making sure to stop and scrape around the bottom and sides of the bowl after each egg.
Add the dry ingredients in three additions alternating with the buttermilk mixture (begin and end with the dry ingredients) on low speed. Mix in each addition until just combined, making sure not to over mix the batter.
Use a rubber spatula to scrape around the bottom and sides of the bowl to ensure that everything is well combined.
Remove 2 cups (430 grams) of vanilla cake batter and transfer to a clean mixing bowl. Add the cocoa powder mixture and gently mix until well combined.
Pour some of the vanilla cake batter into each pan and spread around into one even layer. Alternate spoonfuls of the chocolate cake batter and remaining vanilla cake batter, making sure to evenly distribute both between the two pans.
Use a knife to gently swirl the batter to create a marble effect. Gently shake or tap the pans on the counter to level out the batter.
Bake for 28 to 33 minutes or until a toothpick inserted into the center of the cakes comes out clean. Remove from the oven and allow to cool in the pans for about 20 minutes. Slide a knife around the outside of the cakes, then carefully remove the cakes from the pans, and transfer to a wire rack to cool completely.