pinch of cream of tartar or use 1/2 tsp lemon juice or vinegar - add it to the egg whites, it helps to beat them to their full volume and also it helps to prevent over beating of meringue.
AMERICAN BUTTERCREAM
5sticksunsalted room temperature butter (565 grams, 1 stick is 113 grams)
7cuppowdered sugar
1tbspVanilla Extract
4tbspmilk, heavy cream
1/8tspsalt
Instructions
SPONGE CAKE
Preheat oven to 350F/180C. Butter and flour 2 - 6 inch baking pans, line the bottom with parchment. Or use nonstick spray and line the bottom with parchment.In a large bowl, whisk together flour (1 and 1/2cup), salt (1/4tsp), and baking powder (1tsp); set asideTIP: You can also use cake flour for a finer crumb if desired.
Prepare Meringue
In a clean bowl, whip egg whites ( 4 egg whites) with a pinch of cream of tartar or 1/2 teaspoon lemon juice/white distilled vinegar. Beat until foamy. Then gradually add 1/2 of the granulated sugar (1/2 cup, 100 grams)). Beat until stiff and shiny peaks are formed. Don't overbeat, or the meringue will be dry.Gently scrape the meringue into another bowl. Set aside. Now we are going to prepare the egg yolk mixture; no need to clean the mixer bowl.TIP: Wipe down the bowl with vinegar to remove any greasy residue.
Egg Yolks and Sugar – Ribbon Stage
Pour 4 egg yolks into the mixing bowl you just used to whip the meringue. It is okay; it doesn't have to be spotless. Add the remaining granulated sugar (1/2 cup, 100 grams) to the egg yolks. Beat using a wire whip attachment. Beat the egg yolks at medium-high speed. Scrap the bowl once or twice to ensure all egg yolks are well blended. Beat until the ribbon stage is reached. About 5-6, minutes. The mixture will be thick, doubled in volume, and pale in color.Gradually in one continuous pour add oil, mix well. The gradually add warm water. Scrape the bowl in between. Beat in vanilla extract and beat for 2 more minutes. Turn off the mixer.
Sponge Cake Batter
Using fine mesh strainer sift about 1/3 of flour mixture into the egg yolk mixture and carefully fold the flour in with a spatula, then add about 1/3 of meringue and fold in. Continue with the rest of the flour and meringue until no streaks are visible. Then make last 3 folds, making sure you go with the spatula all the way to the bottom of the bowl. Be gentle and don't over mix. You are looking for a light and airy batter.
Bake
Fill your prepared baking pans and bake until a toothpick inserted in the center comes out clean, about 25-30 minutes. Let the cake cool for 5 minutes in the pan. Invert it on a cooling rack and let it cool completely. Wrap them in a food-safe plastic wrap.
Storage
Store undecorated/unfilled cake covered in the fridge for up to 5 days until ready to use.
AMERICAN BUTTERCREAM
On low speed and with a paddle attachment, beat room temperature butter (5 sticks, 565 grams) until smooth, about 2 minutes. Then gradually add powdered sugar (7 cups), 1 cup at a time.Once the powdered sugar is mixed, increase the speed to medium and beat well for about 2-3 minutes. Then add Vanilla Extract (1 tablespoon) and heavy cream/milk/ (2 to 4 tablespoons). You can add more cream if the icing is too stiff and thinner icing is desired. Whip until smooth and silky.TIP: If your powdered sugar has lumps, sift it before using it.
Divide buttercream into 7 portions. Color 6 with rainbow food gel colors. And continue cake decorating process using step by step tutorial listed in the blog post or watch the video.
Notes
CAN I MAKE THIS CAKE AHEAD OF TIME?
Absolutely. Make the cake 3-4 ahead, store in the airtight cake cover in the fridge until ready to serve. If preparing cake ahead of time it might be a good idea to use some simple syrup on cake layer, to keep them moist.
CAN I KEEP THIS CAKE AT ROOM TEMPERATURE?
It can be kept at room temperature for up to 2 days.