Make the Funfetti Cake: Preheat your oven to 170°C (340°F). Grease and line six 6-inch cake pans with parchment circles. This ensures that your cake layers will come out smoothly without sticking. You can also lightly grease the sides of the pans for easy release after baking.
In a large bowl, whisk together the flour, sugar, baking powder, baking soda, and salt. This dry mixture will provide the structure and lift for the cake, so be sure to whisk thoroughly to combine all the dry ingredients evenly.
Next, add the softened butter to the dry ingredients. Mix the butter into the dry ingredients until the mixture resembles soft crumbs. This step helps incorporate the butter evenly into the flour, which will result in a tender, moist cake texture.
In a separate bowl, mix the milk, vegetable oil, egg whites (or whole eggs + egg whites), and vanilla extract. The combination of milk and oil ensures a moist cake, while the egg whites add lightness and airiness to the texture.
Pour the wet ingredients into the dry ingredients and beat until the batter becomes smooth and creamy. You can use a hand mixer or stand mixer on medium speed for this step. Be careful not to overmix, as this can cause the cake to become dense.
Fold in the sprinkles: Gently fold the rainbow sprinkles into the batter using a spatula. This helps distribute the sprinkles evenly throughout the batter without causing the colors to bleed. Be sure not to overmix, as you want to maintain the vibrant colors of the sprinkles.
Divide and bake: Divide the batter evenly between the six prepared pans. Fill each pan about halfway, leaving enough room for the cake to rise. Bake for 18–22 minutes, or until the cakes spring back when lightly touched and a toothpick inserted into the center comes out clean. The exact baking time may vary depending on your oven, so check around 18 minutes and continue baking as needed.
Allow the cakes to cool in the pans for 5 minutes before transferring them to a wire rack to cool completely. It’s important to cool the cakes completely before frosting to prevent the frosting from melting.
Make the Buttercream: While the cakes are cooling, prepare the buttercream. Beat the softened butter in a large mixing bowl until pale and fluffy. This step usually takes about 3-4 minutes on medium speed and creates a smooth, light texture for the frosting.
Gradually add the powdered sugar, one cup at a time, while mixing. This will help incorporate the sugar smoothly without creating a mess. Beat the mixture until the buttercream is light and fluffy, ensuring it’s well combined.
Pour in the milk and vanilla extract, then whip until the buttercream becomes light, creamy, and airy. If the frosting is too thick, you can add more milk, one tablespoon at a time, until it reaches your desired consistency. Add a pinch of salt to balance the sweetness of the frosting.
Assemble the cake: Once the cake layers have cooled completely, level the tops of each layer with a serrated knife if needed to create even layers. Place the first cake layer on a cake board or serving platter and spread a thin layer of buttercream over the top. Repeat the process with the remaining layers, spreading buttercream between each layer. Once all the layers are stacked, apply a thin crumb coat of frosting around the sides and top of the cake to seal in any crumbs.
Chill and apply the final layer: Chill the cake for 20 minutes to allow the crumb coat to set. Afterward, apply a final smooth layer of buttercream around the entire cake, smoothing out the sides and top with an offset spatula or a bench scraper. This creates a clean, professional finish to your cake.
Decorate: Decorate the top and base of the cake with additional rainbow sprinkles for a festive, colorful finish. This adds an extra pop of color and texture to the cake.
