Rainbow vertical cake

Ingredients:

Cakes

• 8 large eggs, separate the whites from the yolks
• 1 cup granulated sugar, divided
• 2 tbsp vanilla extract
• 1 and 3/4 cups cake flour
• 0.5 tsp coarse salt
• 1/4 cup powdered sugar
• Cooking spray
• Special equipment: 2 45x32 cm molds, 2 large kitchen towels and a stand mixer with a heat-resistant bowl, 6 pastry bags

Glaze

• 2 and 1/4 cups granulated sugar
• 2 tablespoons freshly squeezed lemon juice
• A pinch of coarse salt
• Whites of 12 large eggs
• 990 g unsalted butter, cut into 4 cm pieces, at room temperature
• 2 tbsp vanilla extract
• Red, orange, yellow, green, blue and purple gel food coloring

Preparation:

1. Position a rack in the middle of the oven and preheat the oven to 350°F (175°C). Spray two 18 x 13-inch (45 x 32 cm) loaf pans with cooking spray and line them with parchment paper. Spray the parchment paper with cooking spray and dust it with flour, shaking off any excess.
2. Cakes:
   
   In a medium bowl, beat the egg yolks with 1/2 cup granulated sugar and vanilla extract with a mixer on medium speed until the yolks form thick ribbons, about 5 minutes. Transfer to a large bowl, wash and wipe the mixer bowl. Add the egg whites and salt, and beat on medium-high speed until foamy, about 1 minute.
3. With the mixer running, slowly add the sugar and beat until stiff, glossy peaks form, about 5 minutes. Using a large spatula, fold the beaten egg whites into the yolks until fully incorporated. Add a quarter of the flour and fold it in until fully incorporated. Repeat 3 more times until all the flour is incorporated.
4. Divide the batter equally between the two prepared pans. Bake until the cakes spring back when lightly pressed, about 12 minutes.
   
   
   When measuring flour, spoon it into a dry measuring cup and remove any excess. Scooping flour directly from the bag with a measuring cup will compact it, resulting in dry baked goods.
5. Dust two clean, large kitchen towels with powdered sugar and invert the cake layers onto the towels, short sides facing you. Working with one cake layer at a time, remove the parchment and roll each cake layer up along the short side. Set aside to cool.
6. Glaze:
   
   Pour a few centimeters of water into a saucepan large enough to fit the bowl of a stand mixer and bring to a boil. Add the granulated sugar, lemon juice, salt, and egg whites to the bowl of the stand mixer and whisk. Place the bowl over the simmering water and whisk until the mixture is warm to the touch and the sugar is completely dissolved. Transfer the bowl to the stand mixer and beat with the whisk attachment at medium-high speed until the whites cool and form stiff peaks, about 10-15 minutes.
7. While whisking, add the butter to the bowl, a couple of pieces at a time, waiting until fully incorporated before adding the next two pieces. Once all the butter has been added, continue whisking at medium-high speed. The mixture will separate at first. Continue whisking until the buttercream is smooth and spreadable, about 5 minutes.
8. Slowly pour in the vanilla extract, whisking until smooth and light, about 5 minutes more. If the icing is very soft or starting to separate, refrigerate it until it sets slightly but is still pliable, then whisk until light and fluffy before using.
9. Divide 3/4 cup of frosting into six small bowls. Tint each portion red, orange, yellow, green, blue, and purple, transfer to piping bags, and set aside. Cover with the remaining white frosting and set aside.
   
   Note
   
   You can make the glaze ahead of time and store it in airtight containers in the refrigerator for up to 3 days. Before using, bring to room temperature and beat with a mixer on medium-high speed until smooth..
10. Assemble the cake:
    
    Unfold the cooled cakes so they lay flat. Transfer them to a cutting board and, using a serrated knife, cut each cake in half lengthwise to create two long strips (4 total). Starting 0.5 cm from the top, pipe an even line of red icing along each cake. Repeat with the remaining flowers.
11. Take one frosted cake layer and roll it into a tight log. Add another frosted cake layer, meeting the ends and folding it around the center log, as if you were rolling a bun. Repeat with the remaining cake layers, trimming the ends as needed. Refrigerate the cake for 30 minutes.
12. Trim the ends of the cake so they are flat and transfer it to a cake plate, standing it on end and securing it to the surface with a spoonful of the remaining white frosting. Spread a thin layer of white frosting over the entire surface of the cake, covering the crumb coat. Refrigerate the cake for 30 minutes to set. The cake will be 15 cm tall.
13. Cover the cake with the remaining white frosting, using a spatula to create swirls on the surface if desired. Decorate the cake with colorful sprinkles, if desired.