Set the oven to 350°F (175°C) and let it fully preheat. Lightly coat a 10-inch round pan with butter or non-stick spray. For easier cleanup and removal, you can line the bottom with parchment paper.

In one bowl, combine the chocolate cake mix with 3 eggs, ½ cup of vegetable oil, and 1 cup of either water or milk. Stir or beat with a mixer on medium speed until smooth and free of lumps.

In a second bowl, do the same with the vanilla cake mix—add 3 eggs, ½ cup of vegetable oil, and 1 cup of water or milk. Mix until well combined and smooth.

Pour the chocolate batter into one side of the prepared pan, and the vanilla batter into the other. For a more defined split, you can place a temporary divider in the middle, such as a piece of foil-wrapped cardboard or parchment, and remove it once both batters are in place.

Use a spatula to smooth out both sides. Place the pan on the center oven rack and bake for 30 to 35 minutes. The cake is done when a toothpick inserted in the center comes out clean.

Let the cake rest in the pan for about 10–15 minutes. Then transfer it to a wire rack and allow it to cool completely.

Once cooled, slice through the center to showcase the chocolate and vanilla sides. You can serve it plain or add frosting as desired.