Preheat your oven to 350°F (175°C). Grease and flour two 8-inch round cake pans. Line the bottoms with parchment paper.

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

In the bowl of a stand mixer fitted with the paddle attachment, cream together the softened butter and granulated sugar on medium-high speed until light and fluffy, about 5-7 minutes. Scrape down the sides of the bowl as needed.

Add the vanilla extract and mix until combined. Gradually add the egg whites, one at a time, beating well after each addition until fully incorporated. Scrape down the sides of the bowl as needed.

With the mixer on low speed, add the dry ingredients in three additions, alternating with the milk in two additions, beginning and ending with the dry ingredients. Mix until just combined, being careful not to overmix. Scrape down the sides of the bowl as needed.

Divide the batter evenly between the prepared cake pans. Bake for 28-33 minutes, or until a wooden skewer inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes before inverting them onto a wire rack to cool completely.

In the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter on medium speed until creamy.

Gradually add the sifted confectioners' sugar, about 1 cup at a time, mixing on low speed until incorporated. Add the salt and optional vanilla extract.

Increase the mixer speed to medium-high and beat until the frosting is light and fluffy, about 3-5 minutes. If the frosting is too thick, add milk 1 tablespoon at a time until the desired consistency is reached.

Once the cakes are completely cooled, frost them with the buttercream.