Preheat the oven to 350. Line 3 8-inch cake pans with parchment paper, then grease and set aside.

In a medium bowl, whisk together the cake flour, salt, baking powder, and baking soda, and set aside.

In a large bowl or the bowl of a stand mixer, beat together the butter and sugar until light and fluffy. Add the eggs, egg whites, and vanilla, and mix until smooth. With the mixer on low, gradually add the dry ingredients, until well combined, scraping down the sides of the bowl as needed. Pour in the buttermilk and mix until combined.

Divide the cake batter evenly between the three prepared cake pans. (If you don’t own three pans, you can bake the batter in batches. Be sure to cool the pan completely, line with parchment, and grease before adding the next batch of cake batter.) Bake 25-30 minutes, until lightly browned around the edges, and a toothpick inserted in the center comes out clean. Remove from oven and let cool completely.

To assemble the cake, level the tops of each of the cake layers with a serrated knife. Place one cake layer down on a cake turntable or serving platter. Spread with one cup of the frosting. Place the second cake layer on top and spread with another cup of frosting. Place the final layer on top, then spread the frosting around the top and sides.

For a smoother look, add a light layer of frosting (known as a crumb coat) around the top and sides of the cake, then refrigerate for 20-30 minutes. Once the cake has chilled slightly, spread a thicker layer of frosting around the sides and over the top, and add dollops of frosting for decoration, if desired. Sprinkle with multicolored sprinkles. Refrigerate any leftovers.