Ingredients

2 1/4 cups (293g) all-purpose flour

2 1/2 tsp baking powder

1/2 tsp salt

1/2 cup (112g) unsalted butter, room temperature

1/2 cup vegetable oil

1 1/2 cups (310g) sugar

4 large eggs

1 tbsp vanilla extract

1 cup (240ml) buttermilk*

2.5 oz bittersweet chocolate (60% cacao), chopped**

1 1/4 cups (280g) butter (for frosting)

1 cup (189g) shortening (for frosting)

8 cups (920g) powdered sugar (for frosting)

2 tsp vanilla extract (for frosting)

1 cup (114g) natural unsweetened cocoa powder (for frosting)

6-8 tbsp (90-120ml) water or milk (for frosting)

Instructions

Step 1: Prepare two 9-inch cake pans by lining the bottoms with parchment paper circles and greasing the sides. Preheat your oven to 350°F (176°C).

Step 2: In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

Step 3: In a large mixing bowl, beat the butter, vegetable oil, and sugar until light and fluffy, about 3-4 minutes. Don’t rush the creaming step for best texture.

Step 4: Add the eggs two at a time, mixing well after each addition. Scrape down the bowl sides to ensure even mixing.

Step 5: Add half of the dry ingredients to the batter and mix just until combined.

Step 6: Mix in the buttermilk and vanilla extract until smooth.

Step 7: Add the remaining dry ingredients and mix gently until smooth, taking care not to overmix. Scrape down the bowl as needed.

Step 8: Set aside 1 1/4 cups of the batter in a separate bowl. Divide the remaining batter evenly between the two prepared cake pans.

Step 9: Melt the chopped bittersweet chocolate, then fold it gently into the reserved batter.

Step 10: Drop spoonfuls of the chocolate batter over the vanilla batter in the pans. Use a knife to swirl the batters together with gentle strokes—once in one direction, then once in the opposite—to create a marbled effect without overmixing.

Step 11: Bake the cakes for 18-22 minutes, or until a toothpick inserted in the center comes out clean.

Step 12: Remove from oven, let cool in the pans for 2-3 minutes, then transfer the cakes to wire racks to cool completely.

Step 13: For the frosting, beat together the butter and shortening until smooth.

Step 14: Add half of the powdered sugar and mix until smooth.

Step 15: Add the vanilla extract, cocoa powder, and 4-5 tablespoons of water or milk, then mix until smooth.

Step 16: Gradually add the remaining powdered sugar, mixing well. Adjust the consistency with additional water or milk as needed.

Step 17: Use a serrated knife to level the cake tops by removing any domes.

Step 18: Place one cake layer on a serving plate or cake board and spread about 1 cup of frosting evenly over the top.

Step 19: Add the second cake layer on top, then frost the outside of the entire cake with the remaining frosting.

Tips & Variations

Use room-temperature ingredients to ensure smooth batter and consistent baking.

For a richer chocolate swirl, swap bittersweet chocolate for semi-sweet or dark chocolate.

Buttermilk can be substituted with 1 cup milk plus 1 tbsp lemon juice or vinegar, let sit 5 minutes before using.

Experiment with adding a pinch of cinnamon or espresso powder to the chocolate batter for added depth of flavor.

Storage

Store the cake covered in an airtight container at room temperature for up to 3 days, or refrigerate for up to 5 days. If refrigerated, bring the cake to room temperature before serving. Leftovers can also be frozen wrapped tightly in plastic wrap and foil for up to 2 months; thaw overnight in the refrigerator and then return to room temperature.

FAQs

Can I use regular milk instead of buttermilk?

Yes, you can substitute buttermilk by mixing 1 cup of milk with 1 tablespoon of lemon juice or vinegar. Let it sit for 5 minutes before adding to the batter.

How do I achieve a good marble effect without overmixing?

Drop spoonfuls of chocolate batter over the vanilla batter, then use a knife to gently swirl the batters with a few strokes in opposite directions. Avoid stirring too much to keep distinct swirls and prevent the colors from fully blending.