Ingredients

For the cake

• 3⅓ cups all‑purpose flour (400g)

• ⅓ cup cornstarch (42g)

• 1 tablespoon baking powder

• ¾ teaspoon salt

• ¾ cup unsalted butter, room temperature (170g)

• 2 cups granulated sugar (400g)

• ⅓ cup vegetable oil (80mL)

• 4 large eggs, room temperature

• 3 large egg yolks, room temperature

• 1 tablespoon vanilla extract

• 1½ cups buttermilk, room temperature (360mL)

For the chocolate buttercream

• 2 cups unsalted butter, divided (450g)

• ¼ cup unsweetened natural cocoa powder (25g)

• ½ teaspoon salt

• ¼ cup whole milk (60mL)

• 5–6 cups powdered sugar (600–720g)

• ½ cup semisweet chocolate chips (90g)

Direction

For the cake

• Preheat oven to 350°F. Grease three 8‑inch pans, line with parchment, and wrap with soaked baking strips if available.

• Whisk flour, cornstarch, baking powder, and salt; set aside.

• Beat butter and sugar on medium for 3 minutes until light and fluffy.

• Add oil and vanilla; mix and scrape bowl.

• Add eggs one at a time, then egg yolks, mixing well after each.

• With mixer on low, add flour mixture in three additions, alternating with buttermilk.

• Scrape bowl as needed; divide batter evenly among pans.

• Bake 25–30 minutes until centers are set and springy.

• Cool in pans 10 minutes, then invert, remove parchment, and cool completely.

For the chocolate buttercream

• Melt ¼ cup butter in microwave; mix in cocoa powder and set aside.

• Melt chocolate chips with milk in microwave, stirring until smooth; cool slightly.

• Beat remaining butter and salt on medium for 1 minute until creamy.

• Add cocoa mixture; mix and scrape bowl.

• Gradually add powdered sugar on low speed.

• Add cooled chocolate ganache; beat until fluffy and spreadable.

• Add milk 1 teaspoon at a time if frosting feels stiff.

For the assembly

• Place first cake layer on stand; spread 1 cup (238g) buttercream evenly.

• Add second layer; repeat frosting.

• Add final layer bottom‑side up for a flat top.

• Cover cake with remaining buttercream; swirl for rustic finish.

• Chill 30 minutes before slicing to set frosting.

Notes

• Cool cake layers completely before frosting — warm layers melt buttercream and cause sliding.

• An offset spatula makes spreading easier and creates smooth or swoopy finishes.

Nutrition

Calories: 1197kcal Carbohydrates: 144g Protein: 10g Fat: 67g Saturated Fat: 37g Polyunsaturated Fat: 7g Monounsaturated Fat: 18g Trans Fat: 2g Cholesterol: 263mg Sodium: 497mg Potassium: 236mg Fiber: 3g Sugar: 104g Vitamin A: 1807IU Calcium: 167mg Iron: 3mg