Preheat oven to 350F (177C). Grease three 62 inch cake pans, line with parchment paper, then grease the parchment paper. Parchment paper helps the small cakes seamlessly release from the pans.
Make the cake: Whisk the cake flour, baking powder, baking soda, and salt together. Set aside.
Using a handheld/stand mixer fitted with a paddle attachment, beat the butter on high speed until smooth and creamy, about 1 minute. Add the sugar and beat on high speed for 2 minutes until creamed together. Scrape down the sides and up the bottom of the bowl with a rubber spatula as needed. Add the egg whites and vanilla extract.
Beat on medium-high speed until combined, then beat in the sour cream. Scrape down the sides and up the bottom of the bowl as needed. With the mixer on low speed, add the dry ingredients until just incorporated. With the mixer still running on low, slowly pour in the milk until combined. Do not over-mix. You may need to whisk it all by hand to make sure there are no lumps at the bottom of the bowl. The batter will be slightly thick.
Pour batter evenly into cake pans. Bake for around 18-21 minutes or until the cakes are baked through. To test for doneness, insert a toothpick into the center of the cake. If it comes out clean, its done. Allow cakes to cool completely in the pans set on a wire rack. The cakes must be completely cool before frosting and assembling
Make the frosting: With a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy, about 2 minutes. Add 4 and 1/2 cups confectioners sugar, the heavy cream, and vanilla extract.
Beat on low speed for 30 seconds, then increase to medium-high speed and beat for 2 full minutes. Add up to 1/2 cup more confectioners sugar if frosting is too thin or another Tablespoon of cream if frosting is too thick. Add a pinch of salt if frosting is too sweet. (I add 1/8 teaspoon salt.)
Decorate the cake: make sure cake is cooled completely before start decorating otherwise buttercream will melt. Spread buttercream in each layer of the cake, on top of the cake and around the cake. Decorate with kitkat, m&m and colourful rice.
Keep in the fridge for few hours to set the cake and it is ready to be served! Yummy!
