Special equipment: three 8-inch, three 6-inch and three 4-inch round cake pans; a piping bag fitted with an open coupler; three 8-inch, three 6-inch and three 4-inch round cake boards; a cake turntable; 10 smoothie straws
For the buttercream: Combine the confectioners' sugar, egg whites, vanilla and salt in the bowl of a stand mixer fitted with the whisk attachment. Combine on low until the sugar is incorporated, then raise the speed to high and whip for 10 minutes. Turn to low speed and add half of the butter to combine, then turn off the mixer and add the remaining butter. Scrape down the sides of the bowl and the whisk, then whip on medium speed for 2 to 3 minutes. Hold at room temperature until using.
For the cake: Preheat oven to 335 degrees F. Spray three 8-inch, three 6-inch and three 4-inch round cake pans with cooking spray and line the bottoms with parchment. Spray the parchment.
Sift 2 3/4 cups cake flour, 2 cups granulated sugar, 2 1/2 teaspoons baking powder, 1 teaspoon salt and 1/4 teaspoon baking soda into the bowl of a stand mixer fitted with the paddle attachment. Combine on low for 30 seconds.
Whisk together 3/4 cup melted butter, 1 cup sour cream, 1/2 cup buttermilk, 1/2 cup oil, 3 eggs, 1 egg yolk, 1 tablespoon vanilla and a third of the zest in a medium bowl.
With the mixer running on low, add about half of the wet ingredients and blend until a smooth paste forms. Scrape down the bowl and paddle with your spatula.
Turn the mixer on low and add the remaining wet ingredients. Raise the mixer speed to medium and beat for 1 full minute.
Repeat 2 more times with the remaining ingredients in the same manner.
Divide the batter among the pans, pouring 26 ounces into each of the 8-inch pans, 17 ounces into each of the 6-inch pans and 8 ounces into each of the 4-inch pans.
Bake until a toothpick inserted in the centers comes out with some moist crumbs adhering, 28 to 32 minutes. Cool in the pans for 10 minutes, then turn the cakes out onto cooling racks and let cool completely, about 1 hour.
Build the cake: Trim off the domed tops from the cake layers using a long, serrated knife. Transfer some of the buttercream to a piping bag fitted with an open coupler. Place one 8-inch layer on an 8-inch cake board and evenly pipe a layer of buttercream across the top. Place a second 8-inch cake layer on top, and repeat the process, topping with the third 8-inch cake layer. Refrigerate until the buttercream is firm, 20 to 30 minutes.
Meanwhile, repeat this process with the 6-inch and 4-inch cake layers.
Place the 8-inch tier on a turntable. Using an offset spatula, cover the tier with a generous amount of buttercream. Use a bench scraper or plastic scraper to carefully scrape away most of the buttercream, leaving behind a "naked" rustic appearance. Refrigerate until the buttercream is firm, 20 to 30 minutes.
Meanwhile, repeat this process with the 6-inch and 4-inch cake tiers.
To stack the cake: Transfer the 8-inch tier to a cake stand or decorative cake board. Trim 6 smoothie straws to the height of the cake and insert them vertically throughout the 8-inch tier (this will provide support for the 6-inch tier, so be sure that they are inserted within 3 inches of the center of the cake). Trim 4 smoothie straws to the height of the cake and insert them throughout the 6-inch tier (within 2 inches of the center of the cake) to provide support for the 4-inch tier. Spread a small amount of buttercream on top of the 8-inch tier to help adhere, and using a large offset spatula, carefully transfer the 6-inch tier to the top of the 8-inch tier. Spread a small amount of buttercream on top of the 6-inch tier to help adhere, and using a large offset spatula, carefully transfer the 4-inch tier to the top of the 6-inch tier.
Decorate with sugar flowers as desired.
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