Confetti and parties go together like cake and confetti frosting!
Adjust oven rack to middle position and heat oven to 350 degrees. Spray inside bottom and sides of each 9-inch round metal cake pan with vegetable oil spray with flour. Line bottom of each cake pan with 9-inch round piece of parchment paper.
In bowl of stand mixer (or large bowl if using handheld mixer), whisk together flour, sugar, baking powder, and salt. Lock bowl into place and attach paddle to stand mixer, if using.
In 4-cup liquid measuring cup (or medium bowl), whisk together milk, egg whites, and vanilla.
Start mixer on low speed. Add softened butter, 1 piece at a time, and mix until only pea-size pieces of butter remain, 1 to 2 minutes.
Slowly pour in milk mixture and mix until combined, about 1 minute. Increase speed to medium-high and beat until mixture is light and fluffy, 1 to 2 minutes. Stop mixer. Remove bowl from stand mixer, if using.
Use rubber spatula to scrape down sides of bowl and stir in any remaining dry flour. Use rubber spatula to divide batter evenly between parchment-lined cake pans and smooth tops (make sure to spread batter out to edges of each pan to create even layer).
Place cake pans in oven. Bake until cakes are golden brown and toothpick inserted in center of each cake comes out clean, 22 to 26 minutes.
Use oven mitts to remove cake pans from oven (ask an adult for help). Place cake pans on cooling rack and let cakes cool completely in pans, about 1 hour. (This is a good time to make the frosting!)
Remove cakes from cake pans and discard parchment (click here to learn how to remove cake from its cake pan).