Tool

• Mixing bowls

• 9×13‑inch cake pan

• Electric mixer

Ingredients

For the cake

• 3 cups all‑purpose flour (360g)

• 2⅔ cups granulated sugar (530g)

• ½ cup Dutch‑process cocoa powder (50g)

• 2 teaspoons baking soda

• 1½ teaspoons baking powder

• ¾ teaspoon salt

• 1 cup whole or low‑fat buttermilk, room temperature (240mL)

• ½ cup sour cream, room temperature (120g)

• 3 large eggs, room temperature

• ¾ cup vegetable oil (180mL)

• 1 tablespoon vanilla extract

• 1½ cups hot coffee or boiling water (360mL)

For the frosting

• 1½ cups unsalted butter, room temperature (340g)

• ¼ cup Dutch‑process cocoa powder (25g)

• ¾ teaspoon salt

• 1 pound powdered sugar (450g)

• 1–3 tablespoons whole milk or heavy cream

• ⅓ cup hot fudge sauce or ganache (100g)

• Optional toppings: sprinkles, chopped nuts, shaved chocolate

Direction

For the cake

• Preheat oven to 350ºF. Grease a 9×13 pan; line with parchment sling if removing from pan.

• Sift flour, sugar, cocoa, baking soda, baking powder, and salt; whisk to combine.

• In another bowl, whisk sour cream, buttermilk, eggs, oil, and vanilla.

• Slowly whisk in hot coffee.

• Pour wet ingredients into dry; whisk until no flour streaks remain.

• Pour batter into prepared pan.

• Bake 45–50 minutes until center feels springy and edges pull from pan.

• Cool completely in the pan.

For the frosting

• Beat butter on medium for 2 minutes until smooth and creamy.

• Add cocoa powder and salt; mix on low until combined.

• Add powdered sugar gradually, ½ cup at a time, scraping bowl as needed.

• Add fudge or ganache; mix on medium‑low until combined.

• Add milk or cream until frosting is smooth, fluffy, and spreadable.

Assemble the cake

• Once cooled, remove cake from pan if desired; discard parchment.

• Spread buttercream over the top using an offset spatula.

• Decorate with sprinkles, nuts, or shaved chocolate.

Notes

• Chocolate cake batter should be thin and pourable — not thick like vanilla cake batter.

• Best doneness test: gently press the center; it should spring back. Toothpicks can cause sinking if used too early.

• Serving sizes:

  • Cut into thirds lengthwise × fifths crosswise = 15 servings

  • Fourths × fifths = 20 servings

  • Thin rectangles can serve up to 30 people

Nutrition

Calories: 676kcal Carbohydrates: 93g Protein: 6g Fat: 33g Saturated Fat: 15g Polyunsaturated Fat: 7g Monounsaturated Fat: 8g Trans Fat: 1g Cholesterol: 87mg Sodium: 489mg Potassium: 178mg Fiber: 2g Sugar: 69g Vitamin A: 672IU Vitamin C: 0.2mg Calcium: 75mg Iron: 2mg