Easy Rainbow Buttercream Cake

5 from 1 vote

Recipe makes one 6 - inch cake

Prep Time30

Cook Time25

Cooling and Decorating1 30

Course: Cake, Dessert

Cuisine: American, French

Keyword: american buttercream, pastel rainbow cake, rainbow birthday cake, rainbow cake

Servings: 10 people

Calories: 350kcal

Author: Hani

4 large eggs, separated and at room temperature
1 1/2 cup all-purpose flour 210 grams
1 cup granulated sugar, divided 200 grams, divided
4 tbsp oil or melted butter, cooled 55 grams
4 tbsp warm water or milk 60-65 grams
1 tsp baking powder
1/4 tsp salt
1 tsp Vanilla Extract
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. or use 1/2 tsp lemon juice or vinegar

Preheat oven to 350°F/180°C. Grease and flour 2 - 6inch baking pans, line the bottoms with parchment.
In a large bowl, whisk together flour (1 and 1/2cup, 210 grams), salt (1/4tsp), and baking powder (1tsp); set aside

In a clean, grease-free 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.

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 ( 4 tablespoons, 55 grams) and mix well. Then, gradually add warm water. Scrape the bowl in between. Beat in the Vanilla extract and beat for 2 more minutes.

Using a fine-mesh strainer, sift about 1/3 of the flour mixture into the egg yolk mixture, carefully fold the flour in with a spatula, then add about 1/3 of the meringue and fold it in. Continue with the rest of the flour and meringue until no streaks are visible. Then make the last 3 folds, making sure you go with the spatula all the way to the bottom of the bowl. Be gentle and don't overmix. You are looking for a light and airy batter.

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.

Undecorated cakes keep well at room temperature, well covered, for up to 3 days.

On low speed and with a paddle attachment, beat room-temperature butter (5 sticks, 565 grams) until smooth, about 2 minutes. Then gradually add sifted powdered sugar (7 cups, 840-875grams), 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 a 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.

Can I make this cake ahead of time? Absolutely. You can make the cake 3-4 days ahead. Store in the airtight cake cover in the fridge until ready to serve. If preparing the cake ahead of time, it might be a good idea to use some simple syrup on the cake layers to keep it moist.
Can I keep this cake at room temperature? It can be kept at room temperature for up to 2 days.
Why do you use cream of tartar, vinegar, and lemon juice in egg whites? It helps unfold tight protein chains, making meringue more stable and voluminous, and it also helps prevent overbeating.