Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans to ensure easy removal after baking.
In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt until well combined.
In a large mixing bowl, beat together the softened unsalted butter, vegetable oil, and sugar until fluffy and light in color.
Add eggs one at a time while mixing well after each addition, then stir in vanilla extract until fully incorporated.
In another bowl, combine the buttermilk and melted bittersweet chocolate until smooth before gradually adding it to the butter mixture.
Divide the batter evenly into two bowls; tint one half with black gel icing color and one with red gel icing color before swirling them together in your prepared pans for a stunning marble effect!