Rainbow Vanilla Cake

• Preheat oven to 350°F (177°C). Meanwhile, grease three 6-inch cake pans, line with parchment paper and grease the parchment paper. Set them aside. Parchment paper helps the cake release easily from the pans.
• In a large bowl, sift together the all-purpose flour, cake flour, baking powder, baking soda and salt. Set aside.
• In a separate bowl, add the unsalted butter and granulated sugar. Using a hand mixer or a stand mixer, beat on medium speed until light and creamy, about 2 minutes. Add in the egg and extra egg yolk. Beat again until combined. The mixture will now be light and fluffy. Lastly, stir in the oil and vanilla. Beat until thoroughly combined.
• Add the dry ingredients into the wet ingredients about 4-5 tablespoons at a time and beat on low speed until combined. Repeat until all dry ingredients are added. Lastly, add in the buttermilk and beat until it is fully incorporated.
• Your cake batter will weigh 1146g at this point. Divide it into three parts, each weighing 382g. Add in the the food coloring until the desired color is reached. 
• Add the colored cake batter into all your cake pans. I don’t recommend letting your cake batter sit after being prepared. This can result in the cracked cake tops.
• Cover your cake tins with cake strips, foil or a damp, wet cloth so that the cakes don’t come out dome-shaped and can be layered easily. Bake for 20-22 minutes or until a toothpick inserted in the center comes out clean. Baking times vary, so keep an eye on yours.
• Remove the cakes from the oven and set on a wire rack. Allow to cool completely in the pan.

Vanilla Buttercream

• In a large bowl, add the unsalted butter. Using a hand mixer or a stand mixer, beat on medium speed until creamy, about 2 minutes.
• Add the powdered sugar in batches, one-third at a time and beat on low speed until it is fully incorporated. Repeat until all the sugar is added. Clean the edges of your bowl using a silicone spatula when done. The mixture will now be light and fluffy.
• Add in the vanilla. Beat on medium speed until combined into the buttercream.

Frosting the cake

• In order to evenly frost your cake, use a cookie scoop to add the prepared buttercream on top. Start by adding around 1 large scoop or ½ cup of frosting on top of the first layer.
• Using an offset spatula, spread the frosting evenly on top of the cake. Repeat with the other two cake layers. 
• Add around another or ½ cup of frosting around the cake using an offset spatula. This will be the crumb coat of the cake. Make sure not to put too much pressure and remove crumbs from the cake. 
• Refrigerate for around 30 minutes t0 allow the crumb coat to set on your cake. 
• Add around another ½ to 1 cup of frosting around the cake using an offset spatula to form a thick layer along the edges of the cake. You can use a bench scraper or an icing scraper to smooth out the walls of the cake.
• Smooth the top and edges using an offset spatula to form sharp corners. Chill the cake in the refrigerator for another 20 minutes.

Rainbow Buttercream & Funfetti Base

• While your cake is chilling, take the remaining buttercream frosting and divide it evenly into 4 parts. You will need around 180g of buttercream with each part being 45g. 
• Add in the liquid/gel coloring to each part respectively. 
• Take one piping bag and cut it open. Add thick strips of each frosting color in the order you desire using a spoon or by piping them. Fold the piping bag like a log from one edge to the other. Transfer this piping bag to another one fitted with a 1M Wilton Star Tip. 
• Keeping your piping bag perpendicular to the top of the frosted cake, pipe circles to get a rope shaped rainbow border on the cake. 
• Finally add a thick layer of funfetti sprinkles along the bottom of the cake. Enjoy!
• Store the cake in an airtight container in the refrigerator for upto a week.