Preheat your oven to 350°F (175°C). Grease two 8-inch round cake pans with baking spray.

In a large mixing bowl, cream together 1 cup of softened butter and 2 cups of sugar using an electric mixer until light and fluffy.

Mix in ¼ cup of vegetable oil and then add 3 large eggs one at a time, mixing thoroughly after each addition.

In a separate bowl, whisk together 2¾ cups of all-purpose flour, 1 teaspoon of baking powder, ½ teaspoon of baking soda, and ½ teaspoon of salt.

Gradually add the dry ingredients to the butter mixture, alternating with ¾ cup of buttermilk, mixing just until combined.

Take 1 cup of the batter out and mix in 2 tablespoons of cocoa powder and 1 tablespoon of buttermilk until fully blended.

Spread a layer of vanilla batter in each prepared pan, dollop chocolate batter, and then add another layer of vanilla batter. Swirl gently for a marbled effect.

Bake for 30-35 minutes, checking for doneness with a toothpick. It should come out clean or with a few crumbs.

Let the cakes cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.

Frost with chocolate buttercream or cream cheese frosting once completely cooled, slice, and serve.