Above all else, preheat the oven to 350 degrees Fahrenheit. Then, grease and line a 9-inch round pan.
Next, whisk in the mixing bowl the flour, baking powder, baking soda, sugar, and salt until well combined.
Then, cream the butter at medium-high speed using the hand or stand mixer’s bowl fitted with a wire whisk attachment. Afterward, gradually add the sugar and beat it until fluffy or for about 2 minutes. After that, using a rubber spatula, scrape down the sides and the bowl’s bottom as often as needed.
Next, set the stand or hand mixer at low speed and slowly add the eggs one at a time, then add the vanilla extract. After that, beat at medium-high speed until combined. Then, scrape down the sides and up the bottom of the bowl as often as needed.
Afterward, place the mixer running on low speed and alternately add the dry ingredients and buttermilk in sequence. Make sure not to overmix.
Then, scoop out 3/4 cup of the batter into another mixing bowl and mix in the cocoa powder. After that, pour the remaining batter into the greased and lined baking pan.
Later on, grab a spoon and drip over the white batter’s top. Moreover, make sure to swirl the chocolate batter into the white batter to incorporate it into a marble effect.
Then, put the baking pan in the preheated oven and bake for 55 minutes. Afterward, remove the pan out of the conventional oven and cool in the pan for 10 minutes. Finally, place on a wire rack to cool down.