Preheat oven to 350 degrees Fahrenheit. Then, grease and line with parchment paper the baking pan. After that, set it aside.
Afterward, place the cake mix in a bowl and whisk in with the salt and baking powder.
Next, put the butter in a bowl of a hand or stand mixer fitted with a wire whisk attachment. Afterward, cream the butter at medium-high speed, then gradually add the sugar and whisk until fluffy for about 2 minutes. After that, scrape down the sides and the bowl’s bottom as often as needed using a rubber spatula.
After that, slowly add the eggs one at a time at a low speed, then mix in the vanilla extract. Afterward, beat at medium-high speed till mixed. Then, as often as needed, scrape down the sides and up the bottom of the bowl.
Next, place the mixer running on low speed and alternately add the cake mix and buttermilk in sequence. Make sure not to scrape the corners of the bowl thoroughly with your rubber spatula and avoid overmixing.
Afterward, scoop out 3/4 cup of the batter into another mixing bowl and mix in the cocoa powder. Then, pour the remaining batter into the greased and lined baking pan.
Next, grab a spoon and drip over the white batter’s top. Remember to swirl the chocolate batter into the white batter to incorporate it into a marble effect.
After that, place the baking pan in the preheated oven and bake for 55 minutes or until a toothpick inserted comes out clean. Finally, remove from the oven and place in a wire rack to completely cool down.