This marble buttercream cake starts with vanilla and chocolate cake batter that's swirled together and covered in creamy vanilla buttercream.
Preheat oven to 350 °F. Spray three 6-inch round cake pans with non-stick spray, line the bottom of each pan with parchment paper and spray again. Set aside.
In a medium-size bowl, whisk and stir together the cake mix, all-purpose flour, and salt. Set aside.
In the stand mixer bowl fitted with a paddle attachment, beat the butter, oil, and sugar on medium-high for 5 minutes.
Add eggs, one at a time to the stand mixer bowl and beat on medium until well incorporated.
Turn the mixer on low and add half of the flour mixture to the butter mixture until fully incorporated.
Add the vanilla and milk and beat on low until incorporated. Scrape down the sides of the bowl. Finish by adding the remaining flour mixture and mix on medium for another 30 seconds.
Divide the cake batter into two mixing bowls, keep one bowl vanilla and add cocoa powder to the second bowl, mix with a rubber spatula until well incorporated.
Evenly distribute the cake batters among the three cake pans by alternating the vanilla and chocolate cake batters and use a long skewer to swirl the vanilla and chocolate cake batters together and create marble cakes.
Bake for about 30-35 minutes or until a toothpick inserted into the center of each cake comes out clean or with just a few moist crumbs.
Remove cakes from the oven and let cool in pans for 10 minutes before inverting onto cooling racks to cool completely. Wrap cakes in plastic wrap and place into the freezer to chill for 30 minutes.
In the stand mixer bowl fitted with the paddle attachment, beat the butter until light and fluffy, about 2 minutes.
Turn the mixer to low and gradually add the powdered sugar, followed by the cream, salt, and vanilla extract. Mix until combined. Mix until incorporated completely. Scrape down sides of the bowl.
Turn the mixer to medium-high and beat the frosting for about 5 minutes until light and fluffy. Turn the mixer on low and mix for 2 minutes to push out any air bubbles.
In two small bowls, color the white vanilla buttercream with a small drop of black gel food coloring. One bowl will be a light grey and the other bowl a medium grey tone. Barely mix the buttercreams, so the colors are still streaked with the white. Set aside.
Layer one marble cake, cut side up onto cake board. Add a 2/3 cup of vanilla buttercream over the cake. Place another cake layer cut side down over the top. Repeat the process for another layer — Crumb coat with white vanilla buttercream and freeze for 30 minutes.
Add a thick layer of white buttercream all over the cake. Use an offset spatula to fill in any gaps with more buttercream. Then use a cake scraper to smooth out the sides of the cake. Use the offset spatula to make the top edges sharp and smooth. Freeze for 20 minutes.
In a medium bowl, add 2 cups white buttercream then add 1 Tablespoon of light grey and medium grey colors. Do NOT mix.
Then on a long piece of acetate, or sturdy wax paper, (the same length/circumference of the cake) use an offset spatula to spread the mainly white buttercream. Take a little of each color from the mixing bowl and gently spread the buttercream onto the acetate. Barely mix the buttercreams when spreading on the acetate but keep the buttercream mainly white. This will create the marbled buttercream effect.
Repeat this process until the whole length of the acetate is covered. Fill in the bottom space of acetate covered in buttercream and allow the top of the acetate rough and organic for a contemporary look.
Then on the exposed buttercream, use an offset spatula or cake scraper to make the buttercream smooth and level. Make sure the buttercream is the same thickness before wrapping around the cake.
Carefully, wrap the sheet of acetate around the chilled cake. Use a cake scraper or fondant smoother to smooth out the marbled buttercream and make it even around the cake. Trim the ends, so the edges match up in the back of the cake.
Freeze the cake for 20 minutes to set the marbled buttercream.
Remove the sheet of acetate, revealing the marbled buttercream. Trim any loose edges and use an offset spatula to fill in any gaps.
In a small bowl, mix 2 teaspoons of gold highlighter luster dust and one drop of clear alcohol or lemon extract. Use a small clean paintbrush to paint the edible gold paint along the edge of the marbled buttercream.
Serve cake at room temperature and enjoy!