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Rainbow Vertical Cake

Not only does this cake get flipped on its side and rolled up for a surprising interior but it also gets the dizzying effect of rainbow colored frosting. This is sure to delight the inner child in anyone.
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  • Level: Intermediate
  • Total: 2 hr 40 min (includes chilling time)
  • Active: 1 hr 30 min
  • Yield: 8-10 servings (one 6-inch cake)
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Ingredients

Cake:

Baking spray

8 large eggs, separated

1 cup granulated sugar, separated

2 tablespoons vanilla extract

1 3/4 cups cake flour (see Cook's Note) 

1/2 teaspoon kosher salt

1/4 cup confectioners' sugar

Frosting:

2 1/4 cup granulated sugar

2 tablespoons freshly squeezed lemon juice

Pinch of kosher salt

12 large egg whites

9 sticks unsalted butter, cut into 1 1/2-inch chunks, at room temperature

2 tablespoons vanilla extract

Red, orange, yellow, green, blue and purple gel food color

Directions

Special equipment:
two 18-by-13-inch half-sheet pans, 2 large kitchen towels and a stand mixer with a heatproof bowl, 6 piping bags
  1. For the cake: Arrange an oven rack in the middle of the oven and preheat the oven to 350 degrees F. Spray two 18-by-13-inch half-sheet pans with baking spray and line with parchment. Coat with baking spray and dust with flour, knocking out any extra.
  2. Whip the egg yolks with 1/2 cup of the granulated sugar and vanilla in a medium bowl using an electric mixer on medium-high speed until thick ribbons form, about 5 minutes. Transfer to a large bowl and rinse and dry the bowl. Add the egg whites and salt and whip on medium high speed until foamy, about 1 minute. With the machine still going slowly add the sugar and beat until glossy stiff peaks form, about 5 minutes. Using a large spatula fold the egg whites in to the yolks until well combined. Add a quarter of the flour and fold it until combined. Repeat 3 more times until all the flour is added. 
  3. Divide the batter evenly between the 2 prepared pans. Bake until lightly springy when touched, about 12 minutes.
  4. Dust 2 clean large kitchen towels with the confectioners' sugar and flip the cakes out onto the towels with the short sides facing you. Working with 1 cake at a time, remove the parchment and roll up each cake starting from the short end. Set aside to cool.
  5. For the frosting: Bring a few inches of water to a boil in a saucepan that can hold the heatproof bowl of a stand mixer above the water.
  6. Add the granulated sugar, lemon juice, salt and egg whites to the stand mixer bowl and whisk together by hand. Set the bowl above the boiling water and whisk until the mixture is warm to the touch and the sugar is completely dissolved. Transfer to the stand mixer fitted with the whisk attachment and beat at medium-high speed until cool and the whites hold stiff peaks, 10 to 15 minutes.
  7. Continue to beat, tossing in the butter a couple of chunks at a time, making sure the pieces are incorporated before adding more. After all the butter is incorporated continue whisking on medium-high speed. The mixture will deflate and appear curdled. Continue whisking until the buttercream comes back together with a smooth and spreadable consistency, about 5 minutes. Slowly drizzle in the vanilla while beating until smooth and light, about 5 minutes more. (If the frosting is very soft or begins to break, refrigerate until set but still spreadable and then beat until light before using.)
  8. Transfer 3/4 cup of the frosting to six small bowls. Color them red, orange,, yellow, green, blue and purple and transfer them to piping bags and set aside. Cover the remaining white frosting and set aside. 
  9. To assemble: Unroll the cooled cake layers so they lay flat. Transfer the cakes to a cutting board using a serrated knife to divide each cake in half lengthwise, creating 2 long strips of cake (4 total). Starting ¼-inch from the top of each piece of cake pipe an even line of red frosting lengthwise on each piece. Repeat with the remaining colors. 
  10. Take one strip of frosted cake and roll it into a tight jelly roll. Add another strip of cake, matching the ends and coiling it around the center roll as you would a cinnamon roll; repeat with the remaining cake strips, trimming the ends to fit as necessary. Chill in the refrigerator for 30 minutes. 
  11. Trim the ends of the cake so they are flat and transfer to a cake board or platter, standing the cake up on one end and securing it to the surface with a dollop of the reserved white frosting. 
  12. Spread a thin layer of the white frosting over the top and sides of the cake for a crumb coat. Chill the cake in the refrigerator for 30 minutes to set the coat. 
  13. Frost the cake with the remaining white frosting, using an offset spatula to create swirls if desired. Decorate the top edge of the cake with sprinkles as desired.

Cook’s Note

When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.) You can make the frostings and store them in airtight containers in the refrigerator up to 3 days in advance. Allow them to come to room temperature and mix on medium-high until smooth before using.