1-Preheat oven to 350°F. Grease and flour three 8-inch round pans, line bottoms with parchment paper, and optionally use cake strips for even layers.

2-Sift together flour, baking powder, and salt; set aside.

3-Beat sugar, butter, and oil on medium speed until smooth and creamy (about 3 minutes).

4-Add egg whites in three parts, mixing on low speed and scraping the bowl occasionally; then mix in vanilla until fluffy (about 20 seconds).

5-Add flour mixture in three parts alternating with buttermilk, mixing on low speed to combine each time; scrape bowl and gently fold in rainbow sprinkles.

6-Divide batter evenly among pans, smooth surfaces, and tap pans to remove air bubbles; bake 32-38 minutes, rotating pans halfway through. The cake is done when the centers spring back to touch.

7-Cool cakes in pans for 10 minutes, then invert onto wire racks to cool completely.

8-For the Frosting:

• Beat butter and salt on medium speed until soft and smooth (about 2 minutes).

• Gradually add powdered sugar on low speed, adding heavy cream one tablespoon at a time to maintain a creamy texture.

• Add vanilla extract and beat on medium speed until smooth and fluffy (about 1-2 minutes).

9-Assembly:

• Place one cake layer on a stand and spread 1 cup of frosting evenly to the edges; repeat with remaining layers.

• Spread the remaining frosting over the top and sides or pipe decoration using a large star tip.

• Press sprinkles around the bottom edges of the cake, pipe dollops of frosting on the top edges, and garnish with extra sprinkles.