Prep: Preheat the oven to 350°F (180°C) and grease your cake pan.
Cream the Butter: Beat the softened butter, oil, and sugar together until creamy and well combined.
Add the Eggs: Add the eggs and beat until the mixture becomes lighter and increases in volume.
Add the Milk: Pour in the milk and mix. The batter may look slightly separated at this point, which is okay.
Add the Dry Ingredients: Add the flour, baking powder, and salt. Mix only until everything comes together, then add the vanilla.
Divide the Batter: Separate the batter into two portions, leaving one as the vanilla batter.
Make the Red Velvet Batter: Add the cocoa powder, apple cider vinegar, and red food coloring to the second portion. Mix until you have an even red color.
Create the Marble Effect: Add a thick line or layer of vanilla batter to the prepared pan, then add some red velvet batter over it. Continue alternating the two batters until the pan is filled.
Bake: Bake at 350°F (180°C) for about 45 minutes, or until a toothpick inserted into the center comes out clean.
Cool: Let the cake cool completely before adding the cream cheese glaze.