• Preheat the oven to 350°F/180°C. Line 3 6-inch cake pans with parchment paper and set aside.
• Add 2 cups flour (reserve ¼ cup), the sugar, baking powder, baking soda, and salt to the bowl of a stand mixer or hand mixer fitted with the paddle attachment. Mix on low speed until combined.
  270 grams all purpose flour, 280 grams granulated sugar, 1 ½ teaspoon baking powder, ½ teaspoon baking soda, ½ teaspoon salt
• Beat in the softened butter on low for 3-5 minutes until it looks like wet sand.
  114 grams unsalted butter
• Add oil, eggs, egg white, and vanilla extract. Mix on low until a thick batter forms.
  114 grams neutral oil, 2 large eggs, 1 egg white, 2 teaspoon vanilla extract
• Next, add the Greek yogurt and stream in the buttermilk. Mix on low until well combined, scraping the sides of the bowl and bottom of the bowl as needed.
  140 grams plain Greek yogurt, 100 grams buttermilk
• Scoop half of the cake batter into a separate medium bowl. Mix in the remaining ¼ cup of flour with a rubber or silicone spatula until well combined and set aside.
• Mix the cocoa powder into the cake batter left in the large bowl on medium speed.
  30 grams dutch processed cocoa powder
• Scoop globs of both batters into the cake pans, alternating between vanilla and chocolate, filling each pan about halfway. Move a toothpick or butter knife back and forth to swirl the batters together, creating a marble effect.
• Bake the cakes for 25-35 minutes or until a toothpick inserted in the center comes out clean with a few moist crumbs. Remove from the oven and let the cakes cool in their pans for 10 minutes before transferring to a wire rack to cool completely.

Vanilla Buttercream Frosting

• Add room temperature butter and salt to the bowl of a stand mixer or hand mixer. Beat on medium-high speed for 15 minutes, stopping to scrape the sides of the bowl halfway through. 
  345 grams unsalted butter, ½ teaspoon salt
• Once the butter is light and smooth, gradually mix in the powdered sugar on low speed, stopping to scrape the sides of the bowl occasionally. Then mix in the vanilla extract on medium-low.
  360 grams powdered sugar, 1 ½ teaspoon vanilla extract

Assembly

• Fit a piping bag with a large piping tip and fill with frosting (not all of it). Place one cake layer on a cake board and spread ½ cup frosting on it. Repeat with the second layer.
• Place the last cake layer with the bottom up so the top is flat. Then spread a thin layer of buttercream (or crumb coat) on top of the cake and all over the sides of the cake. Smooth out the frosting with a bench scraper and chill in the refrigerator for 20 minutes.
• Spread the rest of the buttercream on top of the cake and use an offset spatula to work it down the sides. Smooth out the edges and clean up the top.
• Pipe swirls of leftover buttercream on top of the cake. Slice and enjoy!