• Preheat the oven to 325 degrees F. Grease and Flour three 8 inch round pans. As an optional step, we also like to line the bottom of our pans with parchment paper.
• In the bowl of your mixer, add the yellow cake mix, all purpose flour, sugar, and pudding mix. Whisk to blend.
• Add the remaining ingredients to the bowl. Mix on low speed for about 30 seconds.
• Scrape down the sides and bottom of the bowl as needed and mix on medium speed for two minutes. The batter should look well blended.

For the Chocolate Batter

• In a small bowl, dissolve the 4 tablespoons of cocoa powder with 4 tablespoons of very hot water and allow to sit for 3 minutes.
• Scoop two cups of yellow cake batter into a small bowl. Add the cocoa mixture and stir until well combined.

Filling the Pans and Marbling

• To marble, we divided about half of the yellow cake batter between the three cake pans. Then, we divided half of the chocolate batter between the three cake pans on top of the yellow cake batter (scoop/drizzle approximately ⅓ cup chocolate batter into each pan).

  Repeat with the second half of the yellow cake batter and the remaining chocolate batter. Run knife through the pans a few times to marble the batter. Be careful not to do too much marbling as the batter may lose the effect. Tap pans on countertop if needed so that the batter will spread out evenly.
• Bake at 325 for approximately 20-25 minutes or until the layers spring back when lightly touched, or a toothpick can be inserted and removed with just a few moist crumbs attached. Baking times may vary depending on your oven and the type of pans used. Dark pans may bake slightly more quickly.
• Allow the baked marble cake layers to cool in their pans on a wire rack for 10 minutes before turning out. Make sure that they are completely cool before frosting!

For the Chocolate Buttercream

• Add butter and mix on low to medium speed until smooth. Add vanilla and blend into the butter. Add powdered sugar, salt, and cocoa powder.
• Add most of the milk. Begin mixing at low speed so you won't have a cloud of powdered sugar in your kitchen.
• Continue to mix adding remaining milk as needed. It may take 5 to 6 minutes to reach the smoothness and consistency you like. It will take longer if using a hand mixer. As you mix the frosting will become very smooth. You can add additional milk (1 Tablespoon at a time) if the frosting is too thick or additional powdered sugar if too soft.

Assembling and Decorating

• Place the first cake layer on the cake base or pedestal. Spread with chocolate buttercream. Repeat for the next two cake layers. Fill in any gaps between the layers with frosting.
• Next, apply a thin layer of buttercream on top and around the sides of the cake. At this point, we like to chill the cake for 15 minutes in the freezer or longer in the refrigerator.
• Apply the next layer of frosting. Decorate however you like! We smoothed around the sides with a bench scraper and added texture with a small offset spatula. We then piped large buttercream shells on top with a 1M piping tip. We began on the outer edge of the cake and worked our way to the center.

Storing and Serving

• The cake is fine for two days at room temperature in an airtight container (and then move to the refrigerator). If your house is very warm, it is best to refrigerate.
• If refrigerating the cake, remove from the refrigerator two hours before serving for best taste and texture, so that the cake and frosting have time to warm and soften.